Air Gap For Dishwasher

All dishwasher drains need an air gap before connecting to the garbage disposal.
This air gap prevents back-flowing the sewage to the dishwasher.

Actual picture of the air gap connection from the dishwasher to the garbage disposal (bottom view of the countertop)

Dishwasher Backs Up

If the dishwasher drain is clogged, soapy water will come out to the front door.
Dishwasher drain means the pipeline between the dishwasher and the air gap.

Most of the dishwasher drain is small and corrugated, so it is easy to clog the small food debris.