This is attached to most sinks yet few home owners know why it’s there and what it does.  It’s called an “Air Gap.”  Basically, it is to prevent dirty and contaminated water from backing up into the water supply through the dishwasher.  It’s required by code in California but some people bypass this requirement and have something called a “High Loop” under their sink.  A High Loop’s operates simply when the dishwasher drain line is attached to the underside of the countertop or sink.  They both accomplish the same thing and are one of several items home inspectors look for when inspecting a home.

Air Gap for Dishwasher Drain

High Loop for Dishwasher Drain