A more thorough explanation:

Definition: A rat is a slang term used to describe a person who is an informant, especially for the police. It can also refer to a person who acts as a decoy for a gambler or swindler, or for the police to help make an arrest. Another term for a police informant is a stool pigeon.

• The police were able to make an arrest thanks to information provided by a rat.
• The gambler used a rat to distract the other players while he cheated.

In the first example, the term "rat" is used to describe a person who provided information to the police, which led to an arrest. In the second example, the term "rat" is used to describe a person who acted as a decoy for a gambler.