It is a text file editor (kind of like Windows' Notepad, but souped up...) that allows you to edit programming source code with greater ease by what is called syntax highlighting.

The editor itself is written in a programming language called Python and it uses something called GTK to actually create the "stuff" that is shown on the screen.

Oh, and it is purposely made for devices -- like the N900 -- that run the Maemo environment.

If you don't know what any of that means, Google it and to be honest you probably don't need PyGTKEditor

Some reading material:
http://en.wikipedia.org/wiki/Text_editor
http://en.wikipedia.org/wiki/Syntax_highlighting
http://en.wikipedia.org/wiki/Python_...ng_language%29
http://en.wikipedia.org/wiki/GTK
http://en.wikipedia.org/wiki/Maemo

Regards