An access token is a digital key used in computer systems to grant permission to access specific resources or services. When a user logs into an application, the system generates an access token that confirms their identity and authorizes them to perform certain actions. This token is often temporary and has a limited lifespan to enhance security. Access tokens are commonly used in web applications and APIs, allowing users to interact with services without needing to repeatedly enter their credentials. They are part of authentication protocols like OAuth and JWT, which help manage user sessions and protect sensitive information.