A reward is something you get for a job well done. If you study hard for that vocabulary test, an "A" might be the reward in your future.
You can use reward as a verb, as when the mayor rewards a firefighter who saves a family from a terrible blaze, or as a noun, to talk about the actual payment or item that's given to the firefighter. People often offer a reward for the return of lost or stolen things, like bicycles or jewelry or dogs. Reward meant "a regarding" or "an observation" in the 1300's, but it soon came to mean "repayment for some service."