It's a snap-on clamp for a M8 smooth rod. Mount that thing on the X-axis of your Prusa It.2 to improve the belt tension.

Choose one of the stl-files. The 7T-file is more springy and works good with PLA. Depending on your printer's calibration you have to change the clearance value in the OpenScad file. In addition you can use a driller or some tape to make the clamp fit.
I printed that part with a 0.3 mm nozzle. Use a M3 blot to tighten it.