The Cue
A pool cue stick is a long, narrow, round stick that has a rubber tip on the end for striking the cue ball. The cue stick is held in both hands, and slid carefully with the back hand until it makes impact with the cue ball.
The Chalk
Chalk is a blue, powdery substance that comes in a small cube that sits on the end of pool tables.
Application
In each piece of chalk, there is a small indenture that the tip of the cue stick is rubbed in to apply the chalk.
Mechanics
Chalk helps add friction to the round, smooth tip without which the cue stick can slide off of the ball and cause a poor shot.
Is it Necessary?
While you don't have to add chalk every time you shoot, it should be applied often for accurate pool shooting.