Product review (5 stars): It's a rinse aid dispenser cap. . .
And it's amazing.  I moved into an apartment and the dispenser cap was missing in the dishwasher.  since the cap was gone, small bits of food had gotten inside the rinse aid compartment and had made the dishwasher smell a bit funky.  Long story short,  I drained the rinse aid compartment and rinsed it, added some rinse aid and closed it up with this here rinse aid cap.  All my problems went away.  And my dishes are sparkling clean.  Thank you, Rinse Aid Dispenser Cap!