A rinse aid is a drying aid that contains surfactants, which are chemicals that reduce the surface tension of water. This prevents water droplets from forming on dishes after the wash cycle. Instead, the water remains as a thin sheet that rolls off of dishes for faster, more thorough drying.
Even soft water can leave behind traces of minerals like limestone and chalk. Besides speeding up drying times, a rinse aid prevents the formation of water spots, which are mineral deposits left behind when water evaporates.
Surfactants are widely used in the making of soaps and pharmaceuticals. These chemicals are also often found in paint, ink, fabric softener, shampoo and toothpaste. Surfactants have applications in modern fire fighting. Over the years, a number of bodies have raised concerns about how surfactants affect the global environment. However, low concentrations of sulfactants seem to create minimal negative impacts. In the aftermath of 2010's Deepwater Horizon oil spill, large amounts of organic surfactants were sprayed into the Gulf of Mexico. It was thought that this tactic could help microbes digest the spilled oil.
Some people use white vinegar as a natural rinse aid alternative. Due to its acidic nature, however, vinegar could possibly cause damage to dishwashers.