A JWT (JSON Web Token) is a token made of a header, data (claims), and a signature. The server verifies the signature to make sure the token has not been modified. The content is encoded, not encrypted: you must not put secrets in it, and you must check the expiration.
Example
eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMiIsImV4cCI6MTcwMDAwMDAwMH0.<signature>