This is a bottom board.
The bottom board goes on the bottom of the hive, under the stack of boxes.
This style of bottom board is a piece of plywood with three raised edges. The fourth side is left open to create the entrance of the hive when boxes are stacked on top. Boxes fit flush on the sides.
Most people use a hive stand to elevate the bottom board off of the ground. We use 4' by 4' wooden shipping pallets, but there are hive stands commercially available. Four hives fit on a standard pallet. Below is a photo of Grandpa Andersen's hive with stand.