Unless it's a code word for something else, an air gap is simply a system that is not connected to another system. You set up a computer and put it in a separate room and do not connect it to the network. If it's being done for the military, you put a guard at a desk in the room in front of it and let him play Solitaire all day. How would that be objectionable?
Maybe they were looking for ways of sending information out of an air gap, such as by using the sound card or the LEDs on the front panel.