Dishwasher - Water in the Bottom of the Tub
Some water (standing water), usually one to two cups, remaining in the bottom of the dishwasher tub is normal. If dirty water is building up over time in the bottom of the dishwasher and your installation does not have an air gap, make sure you have a high drain loop.
A high drain loop is created when the drain hose is installed with a loop that is a minimum of 32 inches above the floor.
This is accomplished by attaching the drain hose to the underside of the countertop. Without a high drain loop or air gap, dirty water from the sink can flow into the dishwasher.
The dishwasher drain hose should be connected to the house drain either directly or through an air gap. Do not plumb other devices into the dishwasher drain hose.
View a short video about the high drain loop: