MD5 Sums are 32 byte character strings that are the result of running the md5sum program against a particular file. Since it is very hard to find two different files that results in same strings, MD5's can be used to determine that the file or iso you downloaded is a bit-for-bit copy of the remote file or iso.
Verify md5sum using linux:
- md5sum -c linux.iso.md5
Create an md5sum using linux:
md5sum linux.iso > linux.iso.md5
Windows users (will to check the md5sum of a Debian ISO file, etc..) can use winmd5sum.