to rat (someone) out means that you are going to tell an authority (teacher, parent, police, etc.) that someone broke the rules, laws, etc. for example: Don't rat me out to the teacher for stealing Sarah's pencils, please! He is going to rat James out to the cops for breaking into the neighbor's house. They want to rat him out for ripping pages out of library books. Usually, we don't say "I'm going to rat (someone) out" because it has strong negative meaning. When referring to ourselves we usually use "to tell on" For example: I'm going to tell on you for ripping my shirt. She told on him for breaking her toys. I hope that helps ;)

To rat out = To tell the proper authorities i mean, mom, dad, police, etc about who did the crime. I thought he would keep it a secret, but he ratted me out to the police! Now I have to go to jail. My brother is going to rat me out to mom and dad. I am going to be in a lot of trouble, they are going to punish me for sure! Do you understand ?

Also, your title has an error. You should say and write "What does (something) mean?" For example: What does "rat me out" mean? :)

to rat (someone) out means that you are going to tell an authority (teacher, parent, police, etc.) that someone broke the rules, laws, etc. for example: Don't rat me out to the teacher for stealing Sarah's pencils, please! He is going to rat James out to the cops for breaking into the neighbor's house. They want to rat him out for ripping pages out of library books. Usually, we don't say "I'm going to rat (someone) out" because it has strong negative meaning. When referring to ourselves we usually use "to tell on" For example: I'm going to tell on you for ripping my shirt. She told on him for breaking her toys. I hope that helps ;)

It means to tell on you. E.g. If you broke the law, and your friend reported you to the police, then your friend "ratted you out."