MD5 Sums are 32 byte character strings that are the result of running the MD5 sum program against a particular file. Since any difference between two files results in two different 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.

This will require you using the Terminal (/Applications/Utilities/Terminal).

• Launch a Terminal and type (or copy/paste):
  openssl md5

• Drag in the terminal window your file icon to set path to your file

• Press Return

Once the program has run, and it will take a few minutes to run on a 640 megabyte file, a 32 digit md5sum will be generated. This sum should be exactly the same as the listed md5 sum for the specific file or iso you downloaded. If the sums are different, then your downloaded file or iso is not an exact copy, and will have to be downloaded again. It's a bother, but it happens. Better to know before you waste time and energy trying to install something that is never going to install.