An impeller is a mechanical device designed to provide a sucking force through its rotation and contact with air or water. Impellers are typically used in dishwasher water and rinse pumps - made from either hard-wearing plastic, nylon or metal.
Some dishwashers have two impellers: one to spray the water during the wash cycle and one to drain the water. Replacing the spray impeller will not fix drainage problems. If your drain impeller is fine, either your drain line is blocked, or your motor has issues.
Impellers wear with age, in particular, bush sleeves can wear and cause deterioration in performance, foreign objects can also damage impellers. In both cases, the item requires replacement.