I actually downloaded it through a torrent client. What is md5sum? How to verify it?
You can verify if the ISO is corrupted or not without generating MD5 hash. Every torrent client has option to check the downloaded data. Right click on the torrent and click "force check data" or "check data". If the file is corrupted, it will show less than 100% complete, like 98% or so. If so, start downloading again. It will be OK.
If you already have removed the torrent, add it again but do not download. Then, overwrite the new ISO with the previously downloaded ISO(the corrupted one). Now do a "check data".
MD5 is an algorithm. MD5sum is a software that calculates MD5 hash of a given file. MD5 hash is a random string of characters.
In Ubuntu website they have generated MD5 hashes of the ISOs and published it. You download a ISO and generate MD5 hash of that file. If the generated hash matches the one published by Ubuntu, the ISO is OK.
Here is how to generate and check MD5 hashes.
and
List of Ubuntu ISOs and their MD5 hashes.