If something is in its prime, it is at its best. The same goes for a person. If you are young and healthy and in the prime of your life, you are ready to take on the world.
Prime often means "main." The prime reason you go to concerts is to see your favorite groups perform. If your mother is angry that you came home late, tell her that this is a prime example of why you need a cell phone — so you can call her. It can also mean "of the highest quality" as in prime beef or flowers. As a verb, prime means to get ready. Hopefully you've primed yourself for your exams. Prime the pump and start the engine!