A checksum is a redundancy check on the data stored, making sure that it is intact. This happens at start up; a scan of the system and testing it against previous boots. If the data is corrput or otherwise different than what the checksum orignally was, a checksum error will occur.
Although a checksum error can occur during anytime, the most common occurance is during start up. During the startup processes of a computer, the BIOS settings, which are stored in a CMOS memory chip, are computed and then checked against the previous value. If the computed values do not match exactly, the computer warns the user with a checksum error message that the data may have been changed or corrupted between startup cycles. Since all BIOS are not the same, some may allow you to continue with the boot process, or some may require you to fix the problem before allowing you to continue.
In my experience, the most common reason for this failure is due to the battery that holds the CMOS information; the data that is scanned and tested. Considering that you are able to continue using your computer without failure, I would suggest replacing that battery. A CMOS battery is a CR2032, and can be bought just about anywhere that sells batteries.