What are zabutons?

Some articles on zabuton, zabutons:

... The person judged to be the most funny by the host gains more and more zabuton (Japanese cushions), until he may be sitting on a stack five, six, or higher ... are judged to be not funny, or whose responses are not quick enough, may have one or more zabuton removed and end up sitting on the floor ... The zabutons used are supposedly extra-heavy ones weighing 4 kg (9 pounds), so even a stack of ten or more will not topple ...