Modern web applications have two parts: frontend and backend. Frontend is what runs on your phone or in the browser. Lets not about it for now, because this article will take too long. I just want you to understand the backend part for now.
Backend is where the heart of your logic is, and where the database lives. Logic and data is a simplified model on how it’s structured, and these days you’re hear people talk about [Model-View-Controller][] concept. Here I will cover Model and Controller only, and will show you which software plugs in to make a backend.
So the browser triggers a request as a part of a user action. Let it be a tap on a button for example. The server will receive this request and the HTTP server will be the entity handling it. It’s a rare case that only server would be involved.
So you may guess it’s where the Historically backend was uniform. For example, you’d have a HTTP server receiving your requests and serving you data.
It’s called a backend because it’s something the user doesn’t see. It’s simpler than anything else around.