Have you ever moved your camera with your mouse, only to find you have changed targets without having chosen to do so?
Mouse looking is when you click your left/right mouse buttons and look around. However Blizzard by default allows up to 775ms between a mouse DOWN, and a mouse UP event before the game no longer considers it a click event. This large time frame means it is very easy to quickly move your camera around to look for purple to avoid standing in, release your mouse button, and have it coincidentally be over a fellow player and end up changing your target.
This addon lets you set the time frame to whatever you think is best. You can even set it all the way down to ZERO, meaning you can completely disable target changing via mouse clicks. It can also be configured to only alter the click time while in combat.