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. This ensures that only someone with the correct key can produce the same MAC, making it difficult for an attacker to alter the message without detection. MACs are commonly used in various security protocols, such as those found in TLS and IPsec. They help ensure that the data received is exactly what was sent, protecting against tampering and ensuring that the sender is legitimate.