These are air vents for the side of older vr headsets like the Rift S. These will help with fogging of lenses by allowing some air to enter the headset while not allowing light in because of the internal geometry. The thinner side is installed to the inside of the goggle for clearance - entire vent size is 8mm tall and 24 across.

You will need to drill a 5/8 (15.8mm) hole in your headsets facial interface to mount these.