Port is a nautical term referring to the left-hand side of a ship when facing towards the bow; an archaic version of the term is larboard. (The equivalent for the right-hand side is "starboard".) The terms are also used for aircraft, spacecraft, and analogous vessels.
A port buoy is a buoy used to guide vessels. The port buoy is one that a vessel must leave to port when passing upstream. Such buoys are usually green or black, with red buoys to starboard.