A JSON Web Token (JWT) is a type of token used to authenticate and authorize access to resources. It consists of a set of claims encoded in a JSON format with standardized fields such as issuer, subject, and expiry. The most common claim found in a JWT is the authentication claim, which verifies that the holder of the token is authenticated. JWTs are often used in web applications and may include a refresh token to generate new access tokens automatically. They are secure and easy to use, making them a popular choice for identity and access management solutions.