This is a camera mount for a HTC Vive controller. With this you can track your camera in your game space and film Mixed Reality videos. You'll need a 1/4" bolt, two washers and two nuts. I wouldn't tighten it up too much because I don't know how strong the Vive controller is. Just get it tight enough so that your camera is snug and doesn't move around. You use one nut to tighten the two parts together and the second to tighten against the camera.
The part with the flange goes on the bottom. Make sure the indentation on the top part is facing up to give you enough room to put the nuts on.
Here is a good example of a mixed reality video, with some instructions on how to set up the software. Here is a good discussion thread that goes into further detail.