API first is one of the most important concepts you should keep in mind while developing a new product or a service, you have to think of it as if it were the user interface for programmers. In fact, APIs are the way how you can expose the functionalities of your products in a distributed big system.

Nowadays, techniques and approaches such as cloud applications and microservices are becoming adopted everywhere, and they suggest a new way to develop software. With microservices, you have to split your application into different independent services; each service contains a specific set of related features of your application, such as taking some Amazon APIs as an example, we can have something like the following:

As you can see, there are different services that need to exchange information and functionalities. All of those have to define a consistent and self-explained API that the other services have to use in order to either perform an operation in another service that it is not able to do or a client-side application will invoke the APIs directly to access some information.

The main understanding behind API first is that before you implement any code or UI design in your system, you should design your API in such a way that even if you don't have any user interface, developers should be able to navigate and access the information and functionality into your application data.