If you go to the Mint Downloads page you will notice a row "MD5" under the release name. eg Cassandra.
so for the edition of mint you have go down to the MD5 row.
this MD5 value will look like a big mess of numbers and letters.
this is a unique value for the iso file used to check if the iso you downloaded is the same as the one on the site.
some cd/dvd burning software will calculate this value for an iso you want to burn. otherwise you need an md5sum application, it is part of all linux systems. if you are using windows google for a windows md5sum application and install it.
so what you do is run the md5sum application on the iso you downloaded.
you check the value given against the one on the site and if they are the same your iso is good to burn. if they are not the same then something went wrong with the download and broke the iso.
this means you have to download the iso again.
hope this helps.