Under the weirwood tree: White Walker supremacists
We can go no further before we talk about Hodor, the gentle giant who was physically incapable of saying as many words as Guardians of the Galaxy's Groot. Fans had always assumed that Hodor was only able to say his name, but when Bran was visiting the past in an episode earlier this season, we learned that his real name was Wylis, and that he had once been a normal person.
So, what was up with all this Hodor nonsense? It turns out that his name is an elided version of "hold the door," the instruction that Meera Reed shouts repeatedly at him as they're trying to escape the army of the dead as the tree cave is crumbling down around them.
But let's go back to the beginning of the episode. Bran accompanies the Three-Eyed Raven deep into the past via greensight and learns that the Children of the Forest, those little imps that look like Lady Gaga had a brood of spider babies, created the White Walkers. Humans were encroaching on their turf, and they were afraid of losing all of their land and sacred trees, so they drove a piece of dragonglass into a man's heart and magically turned him into a White Walker.