To rephrase something is to say it again, in a slightly different way. You might rephrase your question if the person you're asking doesn't understand it the first time.
You might want to rephrase something to clarify a statement, adjust your ideas to fit your audience, or try not to offend the person you're talking to. A salesman might rephrase his pitch if he's not having much success selling encyclopedias, and an author might rephrase material from a book so that the kindergartners she's talking to will understand her. The word combines the prefix re-, "again," and phrase, from the Greek phrasis, or "speech."