(The following is a general answer and not intended to insult your intelligence)
The air gap is the silver "knob" thing on your sink deck with the little vents in it. Even though they don't look pretty, the vents need to face the sink bowl or else when the vent does overflow, it will do it all over the counter and onto your hardwood floor. (this is BAD)
The Air Gap is there to prevent a phenonemon called "cross connection" which is a situation where a fresh water source supplies water to an appliance below the counter elevation. To prevent a negative water pressure situation from drawing contaminated waste water into the fresh water system, the air gap is required to provide a syphon break to the sewer line.
Common causes of air gap over flows are: 1. Not rinsing the food off your dishes before you put them in the dishwasher. The food particles can clog the waste line. 2. A full and not draining garbage disposal. 3. A new garbage disposal where the homeowner forgot to punch out the knockout plug in the disposal before connecting the dishwasher waste line. 4. Excessive scale in the dishwasher waste line. This can be caused by using too much powdered detergent, which is bad anyway because it etches your glassware. 5. An improperly routed waste line. Look for excessive length or a kinked hose.