A session token is a unique digital code generated by a website or application to identify and authenticate a user during a session, such as when they log in. It allows users to stay logged in and interact securely without repeatedly entering their credentials. The token is typically stored in the user's browser or device and sent with each request to confirm their identity. If a session token is intercepted or stolen by an attacker, it can be misused to access the user's account without needing their password, making its protection crucial for online security.