this is an interactive python shell. you can enter python commands there and immediately see the results. you could use it as a powerful pocket calculator, for quickly trying out short code snippets, or more experienced users can open a python shell for interacting with their running program (think of execute() in c-script just much more powerful).

IDLE by default starts with just this shell. you can open a new empty source code file with "file -> new window". enter your program there, save it and run it with f5.