Aren't is a contraction, a shortened way of saying “are not.” When you squeeze two words together and replace some letters with an apostrophe, you create a contraction. In this case, “are not” becomes “aren't.”

You use aren't when talking about more than one person or thing: “They aren't ready yet” or “We aren't going to the store today.” You also use it with the word “you,” even when talking to just one person: “You aren't listening to me.”

Here's something interesting: in questions about yourself, you might hear people say “Aren't I?” even though it sounds a bit odd. That's because “am I not?” or “amn't I?” sound even stranger, so English speakers settled on aren't as the least awkward option. You might ask, “Aren't I supposed to be on the blue team?”

Like other contractions (such as don't, won't, and can't), aren't makes speech and casual writing feel more natural and conversational. You'll see it constantly in dialogue and everyday conversation, though in very formal writing, people sometimes spell out “are not” instead.