Sake lees. 酒粕.
At the end of the fermentation process of sake, the moromi is squeezed through a fine mesh. The clear sake will flow out and leave a solid rice substance behind. These "dregs" can be as much as one fourth of total volume.
Happily, these sake lees are not wasted. They are used in the Japanese kitchen as a marinade for fish or meat, made into a sweet rice drink (amazake), or re-used for brewing table sake.
Sake kasu are also used as a pickling agent.
[Sakekasu from Daishichi]
In fact, sake kasu are rich in proteins and very nutritious. They take the form of a thick rice paste sold in plastic bags from which the fragrance of sake still wafts up. Another type is sold in dry form. A more expensive kind are sake lees made from Ginjo sake.
In Japan, you will especially find them in winter, when all breweries are operating. Sake kasu has a distinct umami taste.