When you reap something, you enjoy the rewards of your efforts. When your great audition gets you cast in your dream role, you reap the benefits of all that rehearsing you did.
Reap can also describe the act of harvesting crops. The farmer will then sell the crop, reaping a profit. Here, reap means the same as gain. You may have heard the expression "what you reap is what you sow," which comes from the New Testament and can apply to farming and everything else — if, for example, you are patient with other people, they in turn will be patient with you.