This is a small spacer to raise the switch for a mouse button. This is .048" thick, but can easily be modified in tinkered or your slicer to the desired thickness. I used this on a cheaper mouse that the button was slightly too low, and wasn't happy with the way it felt. Simply, desolder the switch, place this spacer over the pins, and solder the switch back in place.
I'm not sure if this is often a problem or not, but hopefully this can help somebody else out!