md5sum is a standard package for most Linux distributions, it is provided by the GNU coreutils - see https://www.gnu.org/software/coreutils/manual/html_node/md5sum-invocation.html
It is used to validate the integrity of the downloaded fusion file. To run execute the md5sum command and pass in the file name as the argument e.g.
$ md5sum exalink_fusion_1.6.1.tar
This output should then be compared to the md5sum reported on the Fusion download page for that version. (https://fusion.exablaze.com/firmware/)
If the md5sum of the downloaded file does not match that on the download page - please retry downloading the file or contact Exablaze for further support.
After verifying the file you can copy it to the Fusion using your preferred option. (https://fusion.exablaze.com/about/firmware_update/)
The tar file contains, internally, SHA256 checksums and an Exablaze signature. In the unlikely case that the tar file is corrupt or tampered,
the firmware update will fail with no changes to the box. So theres no need to manually verify the MD5 checksum.
The update process takes a few minutes and you can then confirm the update has taken place by running 'show version'