Simply put, if you change how much time something in your project takes up on the timeline, ripple adjusts everything after that to compensate. If you shorted or delete an event, rippling will move everything after it over to the left so that you aren't left with a gap. If you lengthen or add something then ripple will move everything after it to the right to make room.

Auto-ripple happens while you edit.

Post ripple is done manually after you edit.