The Daler-Rowney StayWet palette works by osmosis. Acrylic colours dry by evaporation, but when applied to the membrane of a Daler-Rowney StayWet palette, evaporating water is replaced by moisture which penetrates from the reservoir paper through the semi-permeable membrane.
This enables you to paint all day with the colours uncovered. When not in use, colours should be covered with the transparent lid to reduce evaporation and leave the acrylic colours moist and workable for hours. Adding a little water to the reservoir will keep the colours moist and workable for longer.