There are two types of bottom board, a screen bottom board and a solid bottom board.
Screen bottom board has a tight wire screen as a floor, this allows you to quickly and easily check for Varroa Mites by examining the thin metal tray that slides under the mesh.
Solid bottom board is completely wood. This means that you can only check varroa mites by doing a mite wash which kills a little bit of bees each time.
This is a screen bottom board. You can tell because it is a little thicker than a solid bottom board because the screen bottom board needs room for the thin metal try to slide in. See Hive Parts for more details on this entire subject or click a link here to find out more about a specific part, Outer Cover (specifically a telescoping top in this picture), Hive topper/Vent box, Inner Cover, Boxes (Brood chamber and honey super together).