The Air Lock is a feature of Moon. Located in various parts around the space station, these rooms are immune to the effects of outer space, and are filled with air once the power is turned on.
Located in between every major area, these rooms contain the P.E.S., allowing players to switch the Hacker equipment out for the P.E.S. to survive the vacuum of space.
One of the air locks, more specifically the one from Tunnel 6 to the Power Room, has one of the Teddy Bears needed to activate the musical Easter egg Coming Home.
The Air Locks generate an invisible field that "holds in" the oxygen when a door is open. Just touching the field seems to be enough for the character to breathe. This also seems to be the case for the field over the hole in the floor in the Receiving Bay.