Hi Kasi,
Basically the checksum is an algoritm to check the integrity of files after downloaded. The checksum is used for all type of files not only for fixpacks.
The supplier gives you a number, you can calculate the checksum of your downloaded file (in this case fixpack) if the checksums are equals the file is ok, if not is corrupted.
en.wikipedia.org/wiki/Checksum A checksum or hash sum is a small-size datum computed from an arbitrary block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one. If the checksums match, the data was likely not accidentally altered. hope this helps.
Regards