HMAC, or Hash-based Message Authentication Code, is a mechanism used to ensure the integrity and authenticity of a message. It combines a cryptographic hash function with a secret key, producing a unique code that verifies the message has not been altered during transmission. This is crucial in secure communications, as it helps prevent tampering and unauthorized access. The process involves taking the original message and the secret key, then applying the hash function to generate the HMAC. When the message is received, the recipient can perform the same operation to check if the HMAC matches, confirming the message's validity and origin.