What technically defines brain "maturity" or "maturation"?

I’ve been reading Time Magazine’s “The Brain: A User’s Manual” and “My Stroke of Insight” by Jill Bolte Taylor and they often refer to brain “maturity”.

For instance, the brain “matures” from back to front, starting with the occipital lobe and progressing toward the amygdala (cue angry teenager!) and finally in the frontal lobe; a process that continues to death.

Well, what the hell is maturity, in terms of physical changes in the brain? They all assume that is understood, and it’s not!