A judge ruled that a corporation was a person for the purposes of granting first amendment rights to run advertisements endorsing or attacking political candidates. The Koch Brothers could then spend an unlimited amount of money attacking any candidates who proposed environmental regulations, for instance.
It essentially shifted the power in America towards whomever had the bigger megaphone and more campaign financing.
I don't have a ton of "legal" analysis but if you'd like partisan analysis of Citizens United I recommend the Libertarians — https://reason.com/tag/citizens-united/
(you will of course notice the usual partisan ramblings you'd expect from an overtly Libertarian outlet, but, well, they're the ones with interest and motivation to cover it).
But the amazing thing is that you can't say "look at this small nonprofit the FEC was trying to shut up" or even a talk that acknowledges the conflicts between what seems fair and enumerated rights. No, no, instead we must bring nonsense memes with negative information: "corporations and people are the same and Koch money has ended our democracy forever".
