Your /etc/fstab file holds all the mount info for your linux installation. e.g. where to mount local drives, how to mount network drives, …
I have a few NFS mounts defined in my fstab file. Sometimes the other device is not on when my Linux box boots and so the mount is not executed. When I eventually get round to turning the other device on I want to be able to mount it without rebooting my linux box.
You do this with a simple