Acrylic paints dry by the water in the paint evaporating. Because acrylic paints dry very fast, particularly in warm climates, you cannot put a lot of acrylic paint on an ordinary palette as it'll most likely dry out before you've finished, wasting the paint. Various art supply companies have produced moisture-retaining acrylic palettes to solve this problem. What they call it varies, for instance Daler-Rowney's is a Stay-Wet Palette and Masterson a Sta-Wet Palette (Buy Direct).
How Does a Moisture-Retaining Palette Work?
The palettes are made from plastic and consist of a base tray with a tight-fitting lid. A wet piece of watercolor paper (or thin sponge) is placed in the base of the tray to serve as the water reservoir. On top of this is a sheet of grease-proof or baking parchment paper, to serve as a membrane to stop all the water going into the paint immediately. You lay your acrylic paints out on top of the grease-proof sheet. As the water in the acrylic paint evaporates, it is replaced by the water being held in the watercolor paper (a process called osmosis), so the paint doesn't dry out as fast as normal.
What you don't want to use for the membrane is freezer paper, that is paper with plastic on one side. You want paper that's waxed or greasy, this reduces the speed at which the water moves through the paper but doesn't stop it totally.