This is a set of clothes or accessories for figures.
From the Good Smile Company website:
A Nendoroid Pouch: Sleeping Bag made with a special design based on the third brother of the Matsuno family, Ichimatsu! The pouch can fit any almost Nendoroid inside and keep it warm and safe! Take your Nendoroid Ichimatsu Matsuno or any other Nendoroid with you wherever you go!