Hasn't is a contraction of the words “has not.” When you squeeze two words together and replace some letters with an apostrophe, you create a contraction. In this case, the “o” in “not” disappears and gets replaced by an apostrophe: has not becomes hasn't.

We use hasn't to say that something has not happened or that someone does not have something. “She hasn't finished her homework yet” means she has not finished it. “The bus hasn't arrived” means it has not arrived. “He hasn't been to the Grand Canyon” means he has not been there.

Contractions like hasn't make English sound more natural and conversational. You might write “has not” in formal writing, but in everyday speech, most people say hasn't. Compare these sentences: “The package has not arrived” versus “The package hasn't arrived.” They mean exactly the same thing, but the second sounds like how people actually talk.

The opposite of hasn't is has. If your friend hasn't seen the new movie, they have not seen it yet. If they has seen it, well, then they've seen it!