An air gap is a device that can go into a spare sink hole. It gets
inserted between a dishwasher and the sink drain. It's purpose is to
provide an overflow and air break so that if the sink drain backs up,
water can;t syphon back into the dishwasher. Most homes don't use
them, instead they just loop the dishwasher drain hose up high under
the counter. Some places may require an air gap per code.
That soap dispenser is a soap dispenser plus air gap so that you can
get both into a single sink hole. If you don't need the air gap, it
will still function as a soap dispenser.