The port replicator (aka docking station) has a second copy of the keyboard, mouse, video, and possibly other I/O ports. You would leave this part on your desk, connected to your regular keyboard, etc. That way, when you want to take your laptop with you, you just have to remove it from the dock rather than having to unplug many cables. Some docking stations also have extra drives (floppy, cd) so they wouldn't take up extra space or weight in the laptop when you are on the road.
