Air-gaps often look like a round metal cylinder at the back ledge area of the kitchen sink. Their purpose is to prevent dirty water and sewage from the garbage disposal or the sinks drain line from flowing back (or getting sucked back) into the dishwasher and contaminating the dishes; a health issue. Air-gaps are a good thing.
Some dishwashers have built-in backflow devices, however if a chunk of food from the dirty dishes gets lodged in the throat of the check valve, then it may fail to operate properly. Also, having a high loop in the discharge line from the dishwasher to the garbage disposal or tail piece may be suggested by some plumbers, but these do not meet most codes and may fail to protect the dishwasher from having contaminated water back flow into it. Basically the air-gap is the best defense.Who to consult? Plumber or qualified handyman.
Home owners will often install an air-gap themselves. Plumbers and qualified handymen can likewise install an air-gap quickly and easily.