MD5 hashes are a way of verifying the integrity of a file and that no one has changed or messed with the file in any way.
You can use the tool md5sum (installed by default on Debian) to create a hash. So for example,
joe@terrarum:~$ md5sum /bin/ls
Now if I were to transfer /bin/ls to another computer, I could run the command again. If the md5sum matches, that means the file has not been tampered with and everything is fine.