The red pill reddit seems to originate from a bunch of nerds who have had some seriously bad experience with women, through either some bad rejections or somehow toxic relationships. Their next move, of course, was to blame the women. All of them. Every single one. So they decide women have all the power in dating, and that all of them are ***** that will drop you at a moment's notice if you don't have "game" or something.
In order to be a man and have success in this market, you need to get "game" which basically seems to involve a load of PUA tactics combined with low key hating whoever it is you're trying to get with. According to these guys, anyway. Of course, naturally hating all women and believing them all to be somehow poisonous grows into a bit of an anti-feminist movement, and from there it's all a bit weird.
More recently the term "red-pill" is generally thrown around to mean accepting some sort of horrible truth like what happened in the Matrix, which I think is where the name for the OG subreddit must've come from.