It’s a futon used for a kotatsu. “Kotatsu” is a low table with heaters. We use a kotatsu-buton to cover over this table to keep heat inside it. There were no central heating systems or heaters in the past in Japan, so people of that time warmed themselves by placing a hibachi filled with charcoal in their rooms.  The ancient kotatsu was a pottery vessel filled with charcoal and covered with a futon.  And a kotatsu with electrical heaters appeared in recent years. The kotatsu comes in a wide variety of sizes ranging from a compact-sized kotatsu for one user to a large-sized one for multiple users like a big family.