ROUGH IT UP. Chalk is applied to the tip of the cue stick, ideally between every shot, to increase the tips friction coefficient so that when it impacts the cue ball on a non-centre hit, no miscue (unintentional slippage between the cue tip and the struck ball) occurs. The quality of chalk varies greatly from brand to brand, which can significantly affect play. High humidity can also impair the effectiveness of chalk. Cue tip chalk is often not actually the substance typically referred to as "chalk," but some proprietary compound, frequently with a silicate base. "Chalk" may also refer to hand chalk, used to lubricate the cue and bridge hand during shooting (many players prefer talcum powder or a slick pool glove because of the long-term abrasive effect of actual carbonate chalk on the shaft of the cue).
Chalk helps prevent the miscuing of ones shot. In the game of pool, when you make a shot, you must leave the cue ball (white ball) in a good position so that you may have a chance to pocket another ball. Sometimes to get this position for the next shot you might have to put what is know as English on the cue ball. English is where you hit the cue ball high or low and left or right. With English you can see how easy it would be for the tip of the cue to slip and miscue when striking the cue ball.