When we see a region of a planet that is not as heavily cratered as other regions, it means that

When we see a region of a planet that is not as heavily cratered as other regions, it means that the surface of that region is younger.

Explanation:

There are a number of processes that affect planetary surfaces like erosion, volcanism, plate tectonics and the amount of craters by impact of meteorites.

The density of craters in a given region suggests the age of the surface. If the surface is saturated by craters then is likely very old because a long time should have passed for the planet to be hit by so many meteorites. If there are very few craters this may indicate that the surface is quite young, because the other processes described above wouldn't have had enough time to erase them. In any case we need to study the change of the cratering rate over time to determine the age of a given surface.