His is a possessive pronoun that shows something belongs to or relates to a male person or animal. When you say “that's his book,” you're indicating the book belongs to him. If someone left their backpack in the classroom, you might say “I think that's his” to identify the owner.

The word works like her for females or their for multiple people or when gender is unknown or irrelevant. You use it naturally all the time: “his jacket,” “his idea,” “his turn,” “his dog.”

You might notice that his serves double duty in a way that her doesn't. While we say “her coat” (possessive) and “the coat is hers” (possessive pronoun showing ownership), we use his in both situations: “his coat” and “the coat is his.”

Writers sometimes stumble over whether to write “a friend of his” or “a friend of him.” The correct form is “a friend of his” because you're talking about relationship. You'd say “one of his friends,” so you also say “a friend of his.”