A Message Authentication Code (MAC) is a short piece of information used to verify the integrity and authenticity of a message. It is generated by applying a cryptographic function to the message and a secret key, ensuring that only someone with the key can create or validate the MAC. This helps to confirm that the message has not been altered during transmission. When a recipient receives a message along with its MAC, they can use the same secret key to generate a MAC for the received message. If the generated MAC matches the one sent, it indicates that the message is authentic and unchanged.