An ISO is a virtual disk image. It's basically a single file that contains an entire CD/DVD (in fact, they are used to make disks of things like games and OSes). It's a CD without the physical disk. Just like a CD, you can access the files off it. When you mount an ISO, it's treated like a CD drive- you can read it, but can't write it.
Once you have the .iso, Download a program to mount it. Daemon Tools or PowerISO(Recommended), after installing one of those programs, right click on the ISO and there should be a menu to Mount it.
@Janar That's right, Windows 10 automatically mount any ISO if you double click on it.
@Trevor. Have you tried re-installing SA-MP?
Note: Some words were removed from some posts due Piracy rules.