A cushion filler is the material used to fill the cushion cover of a couch or a chair. It is the material that provides the cushioning effect and makes the seating comfortable. The most common cushion fillers include foam, down, polyester, memory foam, and microbeads. The type of cushion filler used can affect the comfort, durability, and maintenance of the cushions. It is important to choose a cushion filler that suits your needs and preferences.