The kotatsu consists of a blanket placed between a low table-frame and table-top, with a heat source placed underneath the blanket. With your legs placed under the blanket, someone wearing traditional Japanese clothing would have warm air come through the bottom of their robes and exit around the neck, heating the whole body.
The origins of the kotatsu can be traced to the 14th century Japanese irori, or cooking hearth. By the 17th century, irori dug into the ground, called hori-gotatsu, resembled a fixed kotatsu. Modern kotatsu, which are moveable, are referred to as oki-gotatsu. Since most Japanese homes have little insulation, kotatsu serve as effective space heaters during the cooler months.