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 algorithm to the message and a secret key. This ensures that only someone with the correct key can produce the same MAC, making it difficult for unauthorized parties to alter the message without detection. When a sender transmits a message along with its MAC, the recipient can use the same key to generate a MAC for the received message. If the two MACs match, it confirms that the message has not been tampered with and is from a legitimate source.