Most terminals attached to hosts are "character mode", each key pressed on the keyboard is sent to the host immediately, with the results echoed back to the user's terminal.
3270 terminals are "block mode". They buffer keyboard entry. When you edit a file in ISPF or a similar product, changes you see on your screen are local to your terminal. When you press the Submit key, the entire screen is sent to the host and updates performed on it are made.
This gives "full screen editing" on IBM mainframes a very different meaning than the one PC users are familiar with.