The kotatsu is a wooden table with an attached heater inside and covered with a blanket. It’s a common way to keep us warm in winter In Japan. Nowadays the heater is electric, but charcoal was used for the original kotatsu in early 1900’s. The electric version was invented in the 1950’s.
One disadvantage of the kotatsu is that once you get in it, you don’t want to get out. People turn into snails…see?
It’s so common that people even make food out of this image. lol