The signatories to the Manifesto all shared a common background in light software development methodologies. The principles they chose reflect this. Again the emphasis is on people-focused outcomes. Each of the following principles supports and elaborates upon the values:

1. Our highest priority is to satisfy the customer through the early and continuous delivery of valuable software: In encouraging incremental delivery as soon and often as we can, we can start to confirm that we are building the right thing. Most people don't know what they want until they see it, and in my experience, use it. Taking this approach garners early feedback and significantly reduces any risk to our customer.
2. Welcome changing requirements, even late in development. Agile processes harness change for the customer's competitive advantage: Instead of locking scope and ignoring evolving business needs, adapt to new discoveries and re-prioritize work to deliver the most value possible for your customer. Imagine a game of soccer where the goal posts keep moving; instead of trying to stop them moving, change the way you play.

3. Deliver working software frequently, from a couple of weeks to a couple of months, with a preference for the shorter timescale: The sooner we deliver, the sooner we get feedback. Not only from our customer that we're building the right thing, but also from our system that we're building it right. Once we get an end-to-end delivery taking place, we can start to iron out problems in our integration and deployment processes.
4. Business people and developers must work together daily throughout the project: To get a good outcome, the customer needs to invest in the building of the software as much as the development team. One of the worst things you can hear from your customer as a software developer is, "You're the expert, you build it." It means that they are about to have very little involvement in the process of creating their software. And yes, while software developers are the experts at building software, and have a neat bunch of processes and tools that do just that, we're not the expert in our customer's domain and we're certainly not able to get inside their heads to truly understand what they need. The closer the customer works with the team, the better the result. 
5. Build projects around motivated individuals. Give them the environment and support they need, and trust them to get the job done: A software development team is a well-educated bunch of problem solvers. We don't want to constrain them by telling them how to do their jobs; the people closest to solving the problem will get the best results. Even the military delegate authority to the people on the frontline because they know if the objective is clear, those people are the ones who can and will get the job done.
6. The most efficient and effective method of conveying information to and within a development team is face-to-face conversation: Face-to-face conversation is a high-bandwidth activity that not only includes words but facial expressions and body language too. It's the fastest way to get information from one human being to another. It's an interactive process that can be used to quickly resolve any ambiguity via questioning. Couple face-to-face conversation with a whiteboard, and you have a powerhouse of understanding between two or more individuals. All other forms of communication dwindle in comparison.
7. Working software is the primary measure of progress: When you think about a software delivery team, and what they are there to do, then there really is nothing else to measure their progress. This principle gives us further guidance around the Agile value working software over comprehensive documentation. 

The emphasis is on working software because we don't want to give any false indicators of progress. For example, if we deliver software that isn't fully tested, then we know that it isn't complete, it has to go through several cycles of testing and fixing. This hasn't moved us any closer to completion of that piece of work because it's still not done.
Done is in the hands of our customer, done is doing the job it was intended to do. Until that point, we aren't 100% sure we've built the right thing, and until that moment we don't have a clear indication of what we might need to redo.
Everything else the software team produces just supports the delivery of the software, from design documents to user guides.

8. Agile processes promote sustainable development. The sponsors, developers, and users should be able to maintain a constant pace indefinitely: Putting a software delivery team under pressure to deliver happens all the time; it shouldn't, but it does. There are a number of consequences of doing this, some of which we discussed earlier in this chapter.

For example, put a team under pressure for long enough, and you'll seriously impact the quality of your product. The team will work long hours, make mistakes, take shortcuts, and so on to get things done for us. The result won't just affect quality, but also the morale of our team, and their productivity. I've seen this happen time and time again; it results in good people leaving along with all the knowledge they've accumulated.

This principle aims to avoid that scenario from happening. Which means that we have to be smart and use alternative ways of getting things done sooner. This means seeking value, ruthless prioritization, delivering working software, a focus on quality, and allowing teams to manage their work in progress so they can avoid multitasking.

Studies have shown that multitasking causes context switching time losses of up to 20%. When you think about it, when you're solving complex problems, the deeper you are into the problem, the longer it takes to regain context when you pick it back up. It's like playing and switching between multiple games of chess. It's not impossible, but it definitely adds time.

I've also seen multitasking defined as messing up multiple things at once.

9. Continuous attention to technical excellence and good design enhances agility: By using solid technical practices and attention to detail when building software, we improve our ability to make enhancements and changes to our software.

For example, Test-Driven Development (TDD) is a practice which is as much about designing our software as it is testing it. It may seem counter-intuitive to use TDD at first, as we're investing time in a practice that seemingly adds to the development time initially. In the long term, however, the improved design of our software and the confidence it gives us to make subsequent changes enhances our agility.

Technical debt is a term first coined by Ward Cunningham. It describes the accumulation of poor design that crops up in code when decisions have been made to implement something quickly. Ward described it as Technical Debt because if you don't pay it back in time, it starts to accumulate. As it accumulates, subsequent changes to the software get harder and harder. What should be a simple change suddenly becomes a major refactor/rewrite to implement.

10. Simplicity—the art of maximizing the amount of work not done—is essential: Building the simplest thing we can to fit the current need prevents defensive programming also known as "future proofing." If we're not sure whether our customer needs something or not, talk to them. If we're building something we're not sure about, we may be solving a problem that we don't have yet.
    Remember the You Ain't Gonna Need It (YAGNI) principle when deciding what to do. If you don't have a hard and fast requirement for it, don't do it.
    One of the number one causes of bugs is complexity in our code. Anything we can do to simplify it will help us reduce bugs and make our code easier to read for others, thus making it less likely that they'll create bugs too.

11. The best architectures, requirements, and designs emerge from self-organizing teams: People nearest to solving the problem are going to find the best solutions. Because of their proximity, they will be able to evolve their solutions so that all aspects of the problem are covered. People at a distance are too removed to make good decisions. Employ smart people, empower them, allow them to self-organize, and you'll be amazed by the results.

12. At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly: This is one of the most important principles in my humble opinion and is also my favorite. A team that takes time to inspect and adapt their approach will identify actions that will allow them to make profound changes to the way they work. The regular interval, for example, every two weeks, gives the team a date in their diary to make time to reflect. This ensures that they create a habit that leads to a continuous improvement mindset. A continuous improvement mindset is what sets a team on the right path to being the best Agile team they can be.