What is a rinse aid?

Rinse aid lowers the surface tension of water so it runs off in a sheet rather than forming droplets that dry into spots. On modern dishwashers, and especially condensation drying models, it is genuinely required for acceptable drying rather than a luxury. An empty rinse aid dispenser is behind a large share of "not drying" service calls, free to check, and it resolves the complaint more often than any part.