A tic is a sudden body movement that a person can't control. For example, it might be a jerking movement of the hand, or blinking a lot, or a throat sound. Even if the person is told that they are doing it, they usually can't stop. Sometimes if they try really hard, they can make the tic go away for a little bit, but they can't make it go away all the time. It's kind of like when you have something in your throat, eventually you just have to cough.
Tics can be either "motor tics" or "phonic tics." If they are "motor tics" it means they're movements, like blinking or shrugging shoulders. If they are "phonic tics" it means they're sounds, like clearing your throat, sniffing, or yelling.