A ‘Stay Wet’ palette is something that is used with acrylic paints to keep the paints moist. Acrylics are extremely fast-drying which means they can dry and stick onto a palette. You often end up scrapping off the acrylic paint from a palette and wasting excess paint.

One way to save this waste of paint is to buy or make a ‘Stay-Wet’ palette. It works by having an absorbent sheet of paper (usually blotting paper) which you wet with water to make moist and a non-porous paper on top (a bit like a greaseproof paper) that the paints are placed on. The palette also has a lid so you can close it to keep the paints moist overnight and between sittings.

You can make your own homemade ‘Stay-Wet’ palette using a plastic tray, blotting paper (or I have seen sponge being used instead) with greaseproof paper on top. I plan to write a post on how to make a Stay-Wet palette in the future.
If you don’t want to buy the palette, you could buy some ‘Tear Off Stay-Wet Palettes’ which come in sheets. They look much like a sketchpad and last a while, the sheets have a non-porous surface that that acrylic paint can sit on and not sink into. These tear off palettes will keep your acrylic paints moist for a long time

If acrylics are your thing then I would recommend getting the Stay-Wet palette with the lid, they aren’t that expensive and are worth it.
Also see Which Palette for Which Paint? and What are Acrylics? for more advice.