1.
mattress futon; bottom futon; sleeping mat
The futon placed on the floor to sleep on, providing cushioning from the hard floor. Contrasts with 掛け布団 (top covering).
敷き布団 is the lower layer of Japanese bedding — the padded mattress futon laid on the floor to sleep on.
Formation: 敷き, from 敷く 'to spread out, to lay down,' + 布団. Literally the 'spread-out futon,' as against the 掛け布団 draped over the sleeper.
Collocations
Cultural note: in traditional Japanese sleeping the 敷き布団 is laid directly on the 畳 at night and stowed in the 押し入れ during the day. That daily cycle is the point: it lets one 部屋 serve as both bedroom and living space, which matters in small homes. Airing the futon regularly is treated as basic housekeeping rather than a chore, since bedding on the floor absorbs moisture.