These are plastic corner pieces that you can use together with 4 mm thick mirror sheets to make a mirror cube or cuboid. Works nicely as a decorative stand/table e.g. as the base of a plant or another decorative item.

Print instructions

The supports are a bit difficult/annoying to remove, but I was able to remove them using just pliers (and safety glasses) and the end result was clean once I was able to get them out.

You may be able to tweak your slicer settings so the supports would be easier to remove, but if you increase the support contact Z distance the slot that needs the supports might become tighter and harder to insert the 4 mm sheet into.