This is just a void block that opens up a hole in the floor. It's good for placing under other items so that 3D grass doesn't come through. It can also be used with the large bases to completely remove the floor and have your map floating in the sky.

Place this file in: Documents\Trackmania\Blocks\

After a restart you'll find the block in the editor in the blocks menu under folder: 7-1-2-3-3-1