What a stupid description.
A checksum means adding each byte in a file and checking it against a verified value, as a simple test for corruption. It's not a very good test, as two or more errors can cancel each other out and the checksum may still pass despite the file being erroneous. An MD5SUM is a better test.
Usually you don't need to worry if a ROM has a bad checksum. It probably just means that something in the ROM was edited for whatever reason, or the manufacturer never gave the game a valid checksum in the game header in the first place.