A biscuit joiner is a woodworking tool used to join two pieces of wood together. In this case, it would be used to join with biscuit #20.
A biscuit joiner uses a small circular saw blade to cut a crescent-shaped hole in the opposite edges of two pieces of wood or wood composite panels.
An oval-shaped, highly-dried, and compressed wooden biscuit (made from beechwood) is covered with glue, or glue is applied in the slot. The biscuit is immediately placed in the slot, and the two boards are clamped together. The wet glue expands the biscuit, further improving the bond.