what is this thing placed on the eyeglass

It’s most likely the temples or arms of the eyeglasses, the side pieces that extend back toward the ears and hold the frame in place. If you mean a small extra part attached to the frame, it could also be a nose pad , hinge , or temple tip depending on where it sits.

Common parts

• Temples/arms : the long side pieces of the frame.

• Temple tips : the ends that rest behind the ears.

• Nose pads : the small pads that sit on the nose, usually on metal frames.

• Hinges : the joints connecting the front of the glasses to the arms.

Best guess

If the object is “placed on the eyeglass” near the side, the most common answer is temple/arm. If it’s something clipped or added on top, then it may be an accessory or a repair part rather than a built-in frame component.

TL;DR: Most likely the temple/arm of the glasses.