It is a checksum which was generated with the MD5-method.
Windows uses CRC, Linux and BSDs (and Mac AFAIK) use MD5.
By getting the MD5 checksum from a program/package/whatever, you can compare it to the checksum of the original, i.e. you downloaded something from a website, and next to it was the checksum. If it is the same, then your download was successful, if not then you need to download it again, since it got damaged during transport.
Ever got the message that the CRC check failed after trying to start a corrupted downloaded exe-file?