A more thorough explanation:

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

Examples:

• After being caught, the criminal decided to become a stool pigeon and give information to the police about his accomplices.
• The gambler used a stool pigeon to distract the other players and cheat at the game.

The first example illustrates the use of stool pigeon as an informant for the police. The criminal decides to become a stool pigeon to provide information to the police about his accomplices in exchange for a reduced sentence or other benefits.

The second example illustrates the use of stool pigeon as a decoy for a gambler. The gambler uses a stool pigeon to distract the other players and cheat at the game. The stool pigeon acts as a decoy to help the gambler win.