I suck at billiards

Touching the ball with your stick to put a mark on it would be considered your turn. The chalk is used to help when you are hitting the cue ball somewhere not in the direct center of it. When you aren't hitting the ball in the middle, the stick wants to just slide off the ball instead of hitting the ball and making it spin. The chalk helps the end of the stick grip to the ball rather than just slide around it. Similar to how you'd put chalk on your hands when wall climbing or doing pull ups, it just gives your hands a better grip.