to rat on/out: (verb)
Situation #1: Two students and a teacher
Maria: Mr. Andersen? I think Hank is cheating off of my test.
Hank: What?! No way!
Mr. Andersen: Let me see your test, Hank.
Hank: Why? This is so unfair.
Mr. Andersen: Hmm... You do have all of the same answers as Maria - even the wrong ones. I'm sorry Hank, but I'm going to have to give you a zero on this test.
Hank: Why did you rat on me, Maria?
Maria: Because you were cheating.
Explanation: To "rat on" or "rat out" someone means to tell a person in a position of authority that someone did something wrong. In the example above, Hank says that Maria "rats on" him when she tells Mr. Andersen that Hank is cheating off of her test. The expression "rat on/out" comes from the American mob, where a "rat" is someone who tells the police secrets; it is a very negative thing to say about a person. "Rat out" is usually interchangeable from "rat on," as can be seen in the next example.
Situation #2: Two friends
Katie: Did you hear about Christina's party last night?
Daniel: Yeah, I went! It was awesome! But it got a little too crazy. There were a lot of drugs being taken.
Katie: What? Did you take any?
Daniel: No! I just had a couple beers.
Katie: Did you call the police?
Daniel: Why would I do that?
Katie: Because people were taking drugs. That's illegal!
Daniel: Well, yeah, but I'm not a rat. I'm not going to rat out my friends just because they decide to something illegal. Especially if they're not hurting anyone.
Katie: Well, I would have called the police.
Daniel: And that's why you don't get invited to parties.