Them is a pronoun we use to refer to people or things we've already mentioned, without repeating their names. If you're talking about your friends and say “I saw them at the park,” everyone knows you mean those specific friends. If you're discussing a stack of books and say “Put them on the shelf,” it's clear you mean the books.

Them works as the object form of “they.” You use “they” when people are doing something (“They arrived early”) and “them” when something is being done to those people or when you're talking about them (“I told them the news” or “This gift is for them”).

The word becomes especially useful when you need to talk about someone without specifying whether they're male or female. If a student left their backpack in the classroom, you might say “Someone needs to return this to them.” This works perfectly whether the owner is a boy or a girl.

Sometimes people informally use “them” in phrases like “That's one of them fancy new phones” instead of “those,” though this isn't standard English. You'll also hear people say things like “It's us versus them,” using them to describe a group that feels separate from your own.