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.
Later, Bran gets bored while the Three-Eyed Raven is zoning out and decides to smoke some trees on his own. He visits the location where the Children of the Forest brewed up that first White Walker, only Bran finds himself in a much more recent time period that stars an undead army instead of Lady Gaga's spider babies. And then he sees the Night's King, whom we can assume is the very first White Walker; worse, the Night's King also sees Bran and grabs his arm.
Apparently, that undoes the magic that protects the cave from the White Walkers and their army of the dead, so the Three-Eyed Raven tells Bran that he has to leave -- but that first, Bran has to "become him." What does that mean? He has to become an old man? He has to become the Highlander of wargs? He has to have a tree growing out of his body like my older brother said would happen to me if I swallowed watermelon seeds?
For some reason, even as the Night's King and company arrive outside the cave and commence knock, knock, knocking on heaven's door, the Three-Eyed Raven takes Bran back to Winterfell in the past, where a preteen Ned Stark is leaving to be fostered in the Vale.
What is so important about the moment in time that the Three-Eyed Raven brings Bran back to? Why doesn't he bring Bran back to the Tower of Joy so we can find out who Jon Snow's mom really is? Why doesn't he take him back to Dallas in 1963 so we can find out what really happened on that grassy knoll? The answer lies with Hodor.