In the ancient past, this area of Mars was bombarded by impactors, forming craters of various sizes in the light-toned material.
Some time after that, a darker material blanketed and covered the field, filling in the craters. Eventually, that blanketed material itself became rock. Long after that, erosive forces (likely wind) acted in this area removing both dark and light-toned material, like an archeologist using a brush to reveal buried structures.
What we see today are a variety of dark circular features that are the remains of the layer that has been eroded back from the walls of the craters that formed them. In some cases, the crater rim is eroded and just a circular dark patch stands on a brighter exposure of rock.
Studying the thickness and characteristics of the dark layer might help scientists learn more about the processes that deposited the material, as well as those that eroded it.
