How do I download the ISO file for a Linux distro from a mirror site? When I open mirrors, it shows a directory with multiple files. What exactly are they?
Mirror sites don't just hold one file, but many. They're a convenience to users, so they can get Ubuntu without delays caused by slow data lines, or delays caused by a mob of users all downloading from the same site at the same time.
The public directory you are pointed in an authorized mirror, such as http://mirror.vutbr.cz/ubuntu/releases/ may have many different distros for Ubuntu, with multiple release numbers, as well as multiple flavors which use different desktop environments, as well as a distro with none at all, the server distro. Distros almost end in .ISO, because they are an 'image' of an entire DVD or CD in one file. Start by picking the version you want, e.g., http://mirror.it.ubc.ca/ubuntu-releases/16.04.4/ and then you will see multiple files, including ISOs.
That directory also has checksums, text files containing a hexadecimal string (like 0123456789ABCDEF), made by other open source programs. When you run a checksum app and point it at the distro you just downloaded, the checksum app gives you a hexadecimal string you compare with the contents of the checksum for the distro.
If they match, then your ISO file arrived intact; if they don't, you need to redownload the ISO because it was corrupted in transit, and installing from it will just waste your time.