A refresh token is a special type of token used in authentication systems to obtain a new access token without requiring the user to log in again. When a user first logs in, they receive both an access token and a refresh token. The access token is used to access protected resources, while the refresh token is stored securely and used to request a new access token when the original one expires. Refresh tokens enhance security and user experience by allowing long-lived sessions. They typically have a longer expiration time than access tokens, enabling users to stay logged in without frequent interruptions. This mechanism is commonly used in applications that rely on OAuth 2.0 and other authentication frameworks.