The TTY window is available to allow the user to pass input to the program being debugged. This window will act similar to the GDB window, except that the data you type will get sent to the program being debugged. See Sending I/O to Inferior.
You will notice that the TTY window has a terminal device between it and the program being debugged. So, if the program being debugged uses say readline, which allows command line editing, the same interface will be provided via the TTY window as at the terminal. You can see the name of the terminal device in the TTY status bar.
The full list of commands that are available in the source window is in TTY Mode.
Sending I/O to the program being debugged can be confusing. It is described better in Sending I/O to Inferior. Unless the I/O with the program being debugged is simple, I usually prefer starting the application on a separate terminal and attaching to it with CGDB.