Kasu refers to all the materials left in the sake squeezing machine after squeezing moromi and is often called sake kasu. It consists of around half and half water (around 50%※1) and rice / alcohol / enzyme mix by volume. Since it contains nutritious elements such as amino acids, vitamins and yeast, kasu is often used as seasoning for Japanese traditional food (such as soup or grilled meat] or even cheese.
Buai means ratio. A higher kasubuai is not economical for the brewery, because it means that there are a lot of possible materials for sake left over. However some sake breweries do squeeze less (leaving a higher kasubuai) in order to achieve a gentle, subtle, smooth sake.
In fact, there is controversial argument that a lower kasubuai does not always means a poorer quality sake, because kasubuai cen be lowered using a higher strength koji enzyme. This stronger enzyme can dissolve more starch (from the rice) into amylase and also keep a relatively higher temperature for activating enzymes at the first stage of brewing.
Reference: ※１：Standard Tables of Food Composition in Japan