An old tribal elder was explaining to children how the world came to be.
Major major story spoilers.
Paraphrasing and remembering these details from yesterday:
She says that in the beginning, there was All-Mother, the Goddess, who created man, beast, and machine. Over time, some humans began to rely on machines serving them. They grew powerful with the machines and built monumental civilizations and creations. The humans who put their faith in the machines are called the faithless. Those who resisted (almost in an Amish-like manner) and aligned with All-Mother, became the Nora tribe (Aloy's tribe) and others you see. This happened when the machines grew so powerful, the faithless were victims to them, and the machines were ruled by a king called the Metal Devil. He plotted to murder All-Mother and her faithful, but the tribes gathered on her mountain and prayed. It was said then that All-Mother struck them down and eradicated the humans of Metal (those who aligned with the machines). But their sins couldn't be eradicated, so ruins of the Metal Era remain strewn throughout the lands. They're considered tainted, and braves from the Aloy tribe are not allowed to visit Metal Ruins or associate with that in anyway, otherwise they are cast out. Some tribes have no problem scavenging Metal treasures in the ruins, however. Because Aloy was apparently born at the site where All-Mother murdered the Metal Devil, Aloy was considered an outcast from birth.
Aloy trains her entire life into adulthood for regaining her brave status in the Nora tribe in a trial called The Proving. Think Hunger Games. Braves train for years to win this competition. Any who survive, including outcasts, are welcomed as braves.
Aloy wins the Proving but almost everyone is immediately ambushed and murdered by outside tribesman who seem to have access to metal technology and are trying to assassinate Aloy because she looks like someone who doesn't seem to fit in this time period to me (not sure).
Rost, the outcast in the intro who raises Aloy into adulthood and realizes he must separate from her once she's a brave, ends up sacrificing himself to save Aloy while he and many others die in the explosion of the ambush.
Other details are unknown to me right now.