means The right-hand side of a ship or aircraft when you're facing forward toward the bow.

from From Old English 'steorbord' — literally 'steer-board.' Before the central stern rudder existed, ships were steered with a large oar or board mounted on the right side of the vessel, handled by most right-handed steersmen. That side became the 'steering board' side. (And because you'd dock with the rudder-free other side against the harbor, that became 'larboard,' later 'port' — but that's a tale for a different word.)