A checksum is the process in which the backup with checksum selected on or archive process will write the data from the tape back to the storage to then compare the results to the original data to ensure that the data has made it to tape in full.
A checksum process may fail if the data did not make it in full.
A checksum process will require that the source storage has enough free space for the data to write back to. If there is not enough space to withhold at least the largest file that is being written, then the checksum will fail.
If a specific set of data was not able to pass checksum, we recommend trying the job again to see if the same data is still not able to checksum and/or the combination of writing just that data seen in the logs that do not pass checksum to see if there is an issue with the data or not.