A docking station is a convenient accessory to have with your laptop. You can connect your external monitor, keyboard, mouse, and other accessories to the docking station. Then all you need to do is attach your laptop to the docking station to make use of all the accessories attached that dock. Then when you need to be mobile again, simply disconnect the laptop from the docking station. This is much more convenient than having to disconnect each device individually from the laptop itself.
A docking station can also extend the laptop interfaces. For example, if your laptop only provides 2 USB ports, the dock could have 4 USB ports that could be used. Additionally, some docking station allow you to add adapter cards for additional functionality.
Docking stations are generally specific to a make and model of laptop, and use a proprietary connector to join the laptop to the docking station.