Relative pronouns are words that are used to link a dependent clause to a main clause. A dependent clause is a group of words with a subject and a verb. It does not express a complete thought so it isn’t a sentence and can’t stand alone.
French relative pronouns can mean who, whom, that, which, whose, where or when. Unlike in French, we don’t always have to use them in English. Many times they are optional. For example, you could say either “the movie I saw last night” or “the movie that I saw last night”.