Hi Cindi,
Lunar libration refers to the wobble that the Moon has. This wobble allows features on the far side of the Moon to become fleetingly visible to us along the limb of the Moon.
This little clip gives an idea of what libration looks like over the course of a lunar 'day':
https://upload.wikimedia.org/wikiped...2007_450px.gif
From the clip, you will see two things: One, the Moon appears to get bigger and smaller - this is because of the elliptical nature of the Moon's orbit; two, along the limb, you will see that features along it come and go from view - this is libration.
Libration is not constant over the lunar cycle. It is cyclical, but its period is much longer than the lunar 'day'. So for instance, to catch a glimpse of Mare Orinetale, the timing to view it is for a few successive months and only for very specific periods of the phase for those months, then the libration cycle takes it out of view for the next few months. Libration has a cycle, and this is what I believe you may be asking about in reference to maximum and minimum libration.
Libration actually allows us to see something like 60% to 65% of the total lunar surface, not just 50% if there was no wobble in the Moon's orbit.
There are special charts to give the timing of when certain far side lunar features can be viewed. Looking at these charts is when the longer period of the whole libration cycle can be noticed. The better quality lunar map apps for smart phones will also show this libration period for far side features.
I hope this answers your question,
Mental.
