When installing a sink,what is air gap.

How are you using the term air gap. Technically the air gap is the space between the faucet spout and the sink. If you are talking about a device called an air gap, it is a device that is used in conjunction with connecting a dishwasher that prevents contaminated waste water from the sink drain flowing into the dishwasher.

Air Gap is a Vacuum Braker device that mounts between - let' say - dishwasher and drain. It is the "air-gap" that will prevent waste water from entering dishwasher.

Commercial faucets have Air Gap built into the spout to prevent dirty sink water to be siphoned back into building's water supply.

I am enclosing two photos of typical Air Gaps: first one is Air Gap used for Dishwashers, the second is Air Gap built-in onto commercial faucets...