The user's intention is expressed through things like mouse clicks and keyboard input. This isn't the most perfect method of communication, so sometimes we have to do a little guessing.
This particular rule means that your program should respond to the user's input in the way the user expects it to respond. Which means that it acts like other things the user has used in the past, or it acts exactly like the documentation says it acts.
Note that I didn't say "other programs the user has used in the past." I said, "other things." Users have used doors in real life, so if your program has a door in it, users expect it to open and close when they push on it or turn the handle. They expect that when the door is open, things can go "through" it, and when the door is closed, things can't go through it.
It is also true for "other programs," though. Users know what a "scrollbar" is because other programs have scrollbars. Users know what a keyboard...