This is for (war)gamers who round off the corners of punched counters using a round corner cutter, as opposed to clipping the corners with a straight edged blade or nail clippers (which leaves them in an irregular octagonal shape).
Here's a comparison of unclipped, straight clipped, and rounded corners:
The corner rounder can be purchased direct from the manufacturer (Oregon Laminations) or from resellers like Amazon in the US. Some European wargame companies sometimes have them in stock too.
The tool comes in a cheaper simpler and a pricier deluxe version. Many find the deluxe easier to use, but others are happy with the simpler version.
The cheaper simpler one hides more of the corner being cut:
The deluxe one shows more of the corner being cut:
The tool comes in several sizes (defined by the radius of the circle, of which a quadrant is being cut off the corner of the counter), and different people prefer different sizes. The sizes 2, 2.5, and 3 mm all are popular.
Here are some sample photos of various sized counters and tool radiuses:
2 mm corner rounder:
The smallest subtlest size. Safest for small counters or counters with info printed close to the corners.
2.5 mm corner rounder:
Some consider it the best "middle ground" size.
3 mm corner rounder:
The largest size in popular use. Some feel the 2 mm & 2.5 mm are too small/subtle for larger counters.