An air gap is a device that acts as a part of the delivery system for your dishwasher drain water to drain more efficiently.
The most common problem with an air gap has to do with water shooting out of it. This will happen when there is an obstruction somewhere in the path of the drain from where it starts at the air gap to where it ends inside your disposal.
If you could lie on your back under your kitchen sink and look up at an air gap, this is what it looks like. There is a white hose coming from the dishwasher and a black hose is going to the disposal. A good plumber will always check the hose to make sure it is clear of any blockage when replacing the disposal.