Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
"md5sum is a computer program that calculates and verifies 128-bit MD5hashes, as described in RFC 1321. The MD5 hash (or checksum) functions as a compact digital fingerprint of a file." ~Wikipedia
In simple words, if you download a Linux Distro and the md5sum result matches the one provided by the web site, your file has practically been downloaded OK.
So, how do we do this md5sum?