So, what is reactive programming? 

It's summarized nicely by the reactive manifesto: https://www.reactivemanifesto.org.

To cite it, reactive programs are:

• Responsive
• Resilient
• Elastic
• Message-driven

To understand these four topics, let's imagine 10 people standing in a line for a cashier. Each one of them can see only the person in front, but not how many people are in the line ahead of that person or what the cashier is doing. Do you have this picture in your mind? Let's start then.