How do I add rinse aid and what does it do?

Rinse aid greatly improves drying and reduces water spots and filming by causing water to sheet off dishes rather than forming droplets. To add liquid rinse aid, turn the dispenser cap 1/4 turn counterclockwise and lift out, then pour in rinse aid until liquid touches the indicated fill level. Replace the cap without overfilling. The dispenser holds enough for 35 to 140 washes depending on the setting.