An MD5 checksum is a 32-character hexadecimal number that is computed on a file. If two files have the same MD5 checksum value, then there is a high probability that the two files are the same.
After downloading an Altera software installation package, you can compute the MD5 checksum on the installation file. Use the computed MD5 checksum to compare against the MD5 checksum provided for that installation file on the download page. By doing this, you can verify the integrity of your download.
There are a variety of MD5 checksum programs available on the Internet.