These are quaity stainless steel p clips/p clamps, with a rubber lining.
They're useful for attaching a rear rack to a frame that has lower eyelets at the dropouts, but no eyelets on the seat stays.
The size refers to the inner diameter. Measure the width of the tube you're clamping with calipers to find the right size. The clamp can be a tiny bit smaller than the tube, but if it's larger it won't clamp on.