A docking station is a unit that attaches to a laptop or notebook computer, either directly or by cable, providing the laptop with more hardware. This arrangement increases the laptop's capabilities almost to that of a standardized desktop computer. A docking station typically includes standard expansion slots, as well as additional ports (sockets) for connecting a printer, modem, keyboard, etc.
So if you have a laptop computer and a docking station, you can take your laptop with you wherever you go, accepting its limitations as a laptop. Then when you come home or go back to the office, you can connect your laptop to the docking station (with some units this is like inserting a giant video tape into a VCR). Then your laptop virtually turns into a regular desktop computer, complete with large monitor, large hard disk, more RAM, etc