When you "map a network drive" you basically assign a drive letter on your computer to the shared resource in the network.
For example. lets say you want to map the C:\share on your WinXP machine, when you map the drive (And enter the correct share name/location \\WinXP\Share) You will have that share available as a drive letter, lets say your E:\ (This could change).
The computer assigns a drive letter to that share for future uses (You dont have to always type in the path).
In my case, I have my server and a few workstations. Win2k Sever, WinXP Pro Workstations (WinXP Home CANT be setup as part of a Domain).
I have the C:\ mapped on my server to all of the workstations on my domain. (Actually its C$ - $ means it is a hidden share, ALL MS Windows OS (Except Win9x) have the C:\ shared by default using the C$ share )
It makes it easy for me to back up data whenever I need to.