You probably mean Rinse Aid - it goes in the small flap usually next to the detergent compartment. Fill it up and close the flap. A little bit is released with each wash - then you fill it up again when the warning light tells you so.
Any decent make of rinse aid will do.
Its job is to allow the rinsing water to drain off the crockery in sheets rather than in droplets - remaining droplets with "bits" in them may otherwise get baked on hard during the drying.
http://en.wikipedia.org/wiki/Dishwasher
Peccavi
May 2011
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