An object pronoun is a type of pronoun that replaces a noun in a sentence, specifically the noun that receives the action of the verb. Common object pronouns include me, you, him, her, it, us, and them. For example, in the sentence "She called me," the pronoun me replaces the noun that is being called. Object pronouns are used to avoid repetition and make sentences clearer. Instead of saying "I saw John and John waved at me," you can say "I saw John and he waved at me." This makes the sentence more concise and easier to understand.