In Unix-like operating systems you 'mount' filesystems, such as hard drives, DVDs, or filesystem images like ISOs with the mount command.
$ mount [whatever]
It's just a word for making a bunch of raw data into something you can navigate and use. You can't really do this in the same way with Windows but people still talk about mounting an image to turn it into a 'virtual drive' at Z: (or whatever) which you can navigate as if it were a physical drive.