Okay, in firefox when you have a web page open, go to View -> Page Source.
This is the "code" if you will. This is what the page contains in order to look and function the way it does. So if you want to build a page, you have to write this code.
Every website, but also every piece of software has what we call "source code" that produces it. So when people like me write software, this is what we write. A collection of source code is a codebase, which can mean for example the entire source code for a program like firefox (this will be probably 50-100mb of text like this).