Here is an image of the Millennium Falcon's docking ring.
A docking ring was essentially a circular, sealed hatch equipped with magnetic couplings on its exterior. This enabled a starship of medium size to connect to a larger capital ship or even a space station. Furthermore, it facilitated connections with a transparisteel gantry. The Millennium Falcon was equipped with a pair of docking rings, one on the port side, and another on the starboard side of the vessel.
