Well, I'm glad you asked.
The iPhone, like any computer, uses a system of files and directories to operate its hardware and its software components. The iPhone uses a modified Unix core and is therefore (after jailbreaking) accessible by many of the same means. The most common and user freindly way is through Secure Shell (SSH).
How to here.
Once in, you can explore and view files and folders that the iPhone uses to communicate and display info. You might want to read a few how-to's and get comfortable with the system structure before you begin making modifications. But you gotta start somewhere!