A kotatsu is a low-in-height, wooden table frame, usually covered by a blanket, upon which a tabletop covered. Underneath the table is a heat source, and the heat source can be electric or charcoal. The kotatsu is mainly popular in Japan, and Japanese culture is very different from Western culture. In western culture, people are not likely to sit on the ground for a long time, but it is pretty regular for Japanese people to do this.
The blanket covered on the table is also called Futon. It is often thick, similar to a quilt, and you can put your lower body under the kotatsu to enjoy the warmth. The traditional Futon is thick and heavy, but there are lighter ones available, either.
Sitting around the kotatsu is an economical and pleasant way to keep the body warm and enhance emotional relationship. It can also serve as a station to work, eat.