You're playing dodge ball during recess. And your friend hurls the ball straight for your face! Quickly, you raise your arm and block it with your elbow.
That was fast! The time it took you to put your arm up is called reaction time. During this time, your body figures out what was happening and how to respond.
Amazingly, all of this happens in under a second!
You can't improve your reflexes because they're hardwired into your system. But you CAN improve your response and your reaction time by doing something over and over again. Even thinking about the task will better your performance.