This book describes the history of the Green Lady.
The Wood Elves are, by nature, secretive and reclusive, yet I do not believe they intentionally hide their lore from outsiders. Take, for example, their practice of choosing new leaders every generation. No one outside the Wood Elves knows exactly how they choose their "Green Lady," but after only a little time I have discovered much about this position, role, and title.
Every generation - a somewhat nebulous concept in itself, there appears no set length of time - a young woman is chosen from among all the Wood Elves to give up her own identity and become the Green Lady. The Green Lady represents, as I understand it, the ferocity, the strength, and the pure physicality of the Bosmer.
Does this mean she is their war-leader or chieftain? Not exactly. Certainly, much of the Green Lady's strength can be turned toward battle, but she is more a manifestation of the physical prowess and even health of her people. I would call her a demigod if I did not think it might offend the true gods as well as the Wood Elves themselves, but it might be an apt description.