These are metal plates bonded to the back of the mirror for hanging purposes. They make it a lot safer and easier to position the mirror than old fashioned wire hangers.
The magnetic discs are fixed to the wall using wall plugs and screws(not supplied), the countersunk screw hole is offset for adjusting the level if the mirror does not look straight. You can simply turn the magnetic fixing before tightening to the preferred position where the mirror will be level when in place. Large mirrors will have 4 metal plates bonded on the back of the mirror 2 attached at the top of the mirror and 2 at the base of the mirror and supplied with 4 magnet fixings, the bottom magnets do not need drilling to the wall. Small mirrors will have 2 metal fixings and rubber supports near the base of the mirror. Please note the mirror will pertrude off the wall the thickness of the plate and magnet fixings which is 15mm. To remove the mirror from the wall simply slide upwards when magnets are free from the metal plates. See Photo.