Message Authentication Code (MAC)

While checksums are useful on their own, they provide no authentication mechanism. For example, if the NSA intercepts a message in transit, they can maliciously alter the message and the checksum. When the message is received by the original recipient, they will think it hasn't been altered.

All a plain checksum does is protect against accidental alterations.

To get the protection we want from malicious attackers, we need to use a MAC, or Message Authentication Code

In short, a MAC is a checksum that is authenticated by a secret key. This means that if the message is altered by someone who doesn't have the secret key, the checksum will not match.