"He, um. He betrays everyone he cares about." It's a huge simplification, but trying to explain Othello to someone that doesn't know it is probably good for his understanding of the play.

"So. So Iago's jealous about a lot of things, like he's been passed over for this promotion and his friend had his love stolen from him, and he agrees to win this girl back for his friend. He sets Othello up that the man he promoted in his place is a drunkard and a fraud, and he's sleeping with Othello's wife. It all gets out of hand, you know, because that's how these plays are. There's a bunch of needless deaths. I won't give away the ending."