The motion in most animation can be broken down in to three sections:
1. Anticipation of an Action - the setup for an action
2. The Action - the actual action
3. The Reaction (Follow Through) - the movement at the end of a motion

Anticipation is the first of the three actions. Although anticipation is not always needed it is usually used, especially in cartoon animation.

There are two main uses of anticipation:
1. Prepare for a movement
2. To draw the viewers attention to something.
The first is the most basic. If you are throwing a ball the anticipation will be bringing your arm back to prepare to throw the ball. Anything done in order to complete an action is anticipation. It's like bringing the roller coaster to the top of the rail with out doing the preparation for the action you don't get an action.

The second is more for the viewer. By drawing the viewers attention to some thing before it is going to happen it helps to keep the audience looking where the animator intends them look. If the character looks off to the side of the screen then the audience will look in that direction. Then you could have something happen like the bad guy could walk in or the character could dodge a shot form the enemy.

There are times when no anticipation is wanted, for example in a scary movie you may not want the audience to know that some thing is going to happen. That way when it happens you get the surprise scare factor in the movie.