It’s a dehumidification sheet used to prevent mold from getting on futons only when futons are laid directly on the wooden floor. In Japan, futons are supposed to be laid directly on the floor, in most cases, on the tatami-mats. But when it comes to the wooden floor, the floor might be soaked with night sweat during sleep, which may cause mold growth. This sheet is used to prevent mold growth and also to prevent the buildup of moisture in oshi-ire. Tatami-mats can absorb the moisture, so you don’t have to use it when you lay out futons on tatami-mats.