Virtual DVD or CD drives are often mounted disk images via disk image emulator ( IE - Daemon Tools ) software. This allows one to read a CD or DVD from the disk image that is usually located on the hard drive, rather than from the disc drive. This allows users to run software requiring the CD or DVD without having to swap discs, or even possess it.
This means that the file that you want to open has to been in a image form like a iso or a bin and cue. Opening the bin file via the cue file. Mounting images would be just like placing disks in the drive. So if you had a pc game that was four images you would go through in order mounting each image one after the other just like if you were changing disks in the drive. The Image would install as normal.
That's about the basics, once you learn that you can start figuring out other tricks.