The Signature verifies that the JWT hasn't been altered.
The Signature part of a JWT is created by encoding the Header and Payload, then signing them with a secret key using a cryptographic algorithm, such as HMAC SHA-256. This ensures that if any part of the JWT is modified, the Signature will not match, alerting the server to possible tampering. For instance, in an e-commerce system, if a user's role in the Payload is changed from 'customer' to 'admin' without proper authorization, the Signature will fail verification, preventing unauthorized access.