The m2 value is a closeness of fit between to the two sets of coordinate points you’re trying to overlap. A better fit = higher m2 value.
Put another way, you can have a significant differences, but the difference is small (say two groups with mean of 1.0 and 1.1, p< 0.05)
or you can have a sig diff with a more obvious difference (mean 1.0 vs mean 100.0, p value < 0.05)
So usually we look for a sig p-value, and M2 > 0.3. Only a guideline, and not likely publishable as gospel.