Its funny name tells what it is. You sit upon it when the ground is damp or dusty.
To make one, first get a large piece of waterproof material, like oilcloth or plastic and cut it into two squares. Next, put a cushion of stacked newspapers [that you have to cut to size] between the squares. Now, fasten together with staples or stitches. Your leader will show you how to make a running stitch or overcast stitch. Be sure to staple or stitch in two pieces of tape to keep the edges from tearing apart.