An Access Token is a digital key used in computer systems to grant permission to access specific resources or services. It is commonly used in authentication processes, allowing users to prove their identity without repeatedly entering their credentials. Access tokens are often generated by systems like OAuth or OpenID Connect after a user successfully logs in. These tokens contain information about the user's identity and the permissions granted. They are typically short-lived for security reasons, meaning they expire after a certain period. When a user wants to access a resource, they present their access token, which the system verifies before granting access.