In regards to plumbing, an air gap is a vertical space between the area where water flows out from a fixture and the flood level of that fixture. A common example of this is the cylindrical fixture often seen mounted on sinks next to the faucet. This air gap works in conjunction with a dishwasher. If you were to look underneath the counter, the dishwasher drain pipe feeds into the top of the air gap, while the bottom of the air gap pumps into the garbage disposal. The air gap prevents dirty sink water from backing up into the dishwasher and clean water lines. Over time, you may need to clean the air gap by removing the cap and using a bottle brush with a generic cleaner to remove food particles and bad smells.