Any application or website that is written is developed using code of a specific language. That code is called the source code. This is the initial state of the application and it can be re-compiled and distributed from that code again.
If you do not have access to the source code. any changes you would like to make would not really be possible unless you find a way to decompile the produced or bundled application and get the source code out of it. This is a very difficult task and sometimes impossible. Source code is what is needed for it as it provides you with the ability to extend updates and extend your app.