1) some kind of foamy stuff.
2) a shallowish plastic container with a lid.
3) Some grease proof paper (called baking paper elsewhere).
You trim the foam to fit into the container, add water and place the baking paper on top, thusly:
Then you press the baking paper down and put dobs of paint on it. It is supposed to keep the paint in a moist and usable state for quite some time. When you take a break you put the lid on and it stays all nice and wet in there.
The report on how it went will come later...