The utility program md5sum was designed to verify data integrity after transmission or copying. It uses the cryptographic MD5-algorithm (Messages digest algorithm 5) to compute a 128-bit long hash value.
The probability of file corruption or truncation increases proportional to the size of a file. It is a very good idea to check large files downloaded using the internet after transmission using the MD5-hash.  So you can be sure the file(s) is/are correct.
On our Download-page for ArtCom-installation ISO image files there is also a matching file with the additional suffix .md5 .
The content of such a file usually looks similar to this:

d3941759813ccfcbb0344c95d4cdfb1d ac21-1-pre_DVD.iso.gz

The string at the beginning is the MD5-hash followed by the filename of the original download file. To check first open a terminal and to the correct download directory.  Than run the command md5sum with the filename of the downloaded file as shown on the following example:

cd Desktop
md5sum ac21-1-pre_DVD.iso.gz

The computation could take a while. Afterwards it sould print out a single line with the same MD5-hash as in the .md5 file. Is this not the case then the file has been damaged during transmission or copying.