I believe it means in animation when an action is exaggerated, and extended for more frames of animation than is natural in order for it be noticed by the audience. The 'overshoot' is the exaggeration, and the 'settle' is the return of a movement to normal. Overshoot and settle is commonly used in action scenes to emphasise movements. An example would be making a character's leg during a kick stay straight for longer, and return to a bent position more slowly. There's a good discussion here: http://www.awn.com/forum/thread/1011924 Hope this helps. :)

It is when s movement goes beyong the final position, and then comes back and settle, If take your arm for example, and move it very fast to any point in the air like throwing a hit you will notice how you can hardly hit your target and immediatly stop at that specific point, What happens its that naturaly your arm will go beyond where you intenfed to stop and then comes back and stop. That is overshoot. It can happens with any movement and different speeds, its just that it is more noticeable as the movement becomes faster or stronger.