During the striking process, if no planchet is between the dies, the die clash together leaving a clash mark from the other die. Often seen around the bust and in the central area of the reverses. Not a big premium, but neat to see and the overlays are showing the mark location on the opposite die that was damage. These dies are often polished on the fields to remove these clash mark. So that is what you are seeing on your coin. Each design will be different, but locate close to the central areas.
CoopHome: What is a die clash mark? How and why is it created? Clash marks.