A small stand for your laptop or tablet. I use it for a Macbook Pro in front of my bike trainer. The recesses are for rubber pads that fit within the feet of the laptop.
The slots allow for different orientation relative to the feet. The through holes were setup for M6-1 screws that fit my Manfrotto tripod, but you may have to adjust the size and diameter for your tripod, it doesn't seem to be universal as it didn't match a Peak Design tripod's set screw holes.