A Signal PIN is a user-chosen secret used by Signal to protect account registration-related features and help preserve account state. It is not the same thing as a chat backup password or a backup recovery key, and it does not by itself decrypt stored message history. In practice, the PIN acts as an extra control around identity and device changes.
This matters because attackers often target the weakest link in a secure app: the user. If someone tricks a victim into sharing a Signal PIN or verification code, the attacker may be able to push a re-registration attempt, lock the legitimate user out, or interfere with account recovery. Defenders should treat any request for a PIN as suspicious, because the PIN is part of the authentication flow, not a helpdesk secret. Its value is strongest when it stays private and is paired with careful handling of recovery information.