When you reread something, you read it for a second (or third, or fourth) time. You may want to reread a book for school to make sure you understand the material before a big test.
If you read the back of a cereal box every morning at breakfast, you'll reread the same text again and again. And, if you reread your chemistry notes a few times before a quiz, it'll help you get a good grade. You probably have a favorite book that you reread every once in a while too. Sometimes people use this word as a noun: "Will you give my story a reread before I submit it to the literary magazine?"