When you explore a web application, do you know what happens behind the scenes? Most likely there is a lot of communication going on. Communication you can inspect and leverage.
What you see is all there is? Have you heard this? It's not quite true when it comes to IT systems. What you're being shown when exploring web applications or systems is just part of the story.
There is a lot of chatting going on in the background. Your web application communicates with dependant services and those services communicate with others. I will show you the most popular ways of communication and what available tools you have in your disposition to inspect and amend what is being exchanged, all that to see how the tested system reacts to changes.
You will also learn about network fragmentation, and how bandwidth, latency, and malformed responses, can affect the way your app behaves.