WARNING: SPOILERS FOR THE 100: THIRTEEN ARE AHEAD. WATCH AT YOUR OWN RISK. I REPEAT: WATCH AT YOUR OWN RISK.
In the previous episode of The 100, we learned that the symbol on the pills Jaha is giving out is the sacred symbol to the Grounders. We learned that it was an artificial intelligence created by someone named Becca. Becca was part of the thirteenth station, Polaris, that the rest of the twelve stations shot out of the sky.
This episode covered a lot of things, one of which was that 97 years ago, Becca was working on a second version of Ally. When Ally determined that the population on Earth was too large, she sent out the missiles that ended the world as they knew it. Becca, who had already made it wrong ONCE, decided to INJECT HERSELF WITH IT LIKE A DUMBASS.
I mean....she injected herself with the untested technology and then she took the escape pod on the station and landed on Earth. Then the twelve stations shot the thirteenth out of the sky. When Polaris, Becca's pod, reached the ground, she emerged in an orange suit with Commander written on it. She rallied the other survivors. This explains why the giant Grounder city is called Polis. The 'a' and 'r' were burnt off upon re-entry. It explains why Polis has a tower: it's part of the thirteenth station.
It also explains the Grounder belief that the spirit of the commander is passed on. When the commander dies, the AI chip is cut out of them and implanted into the successor.
What was the detail that stuck with you the most from this episode (keep in mind that people might not have watched and be nice and relatively spoiler-free of the big stuff.)