Pretty much end-game toxic masculinity - men blaming women for not wanting to sleep with them and promoting "pick-up artist" strategies like negging, gas-lighting, and other despicable behavior in an attempt to get laid. It grew into a fairly large subreddit.

It is a movement glorifying the "alpha male" idea. The strongest man rules and get all the women.

It is called "red pill" because of the idea that it is the real truth of this world.

There's a sub-culture of men who identify as "The Red Pill" who harbor some, uh, non-mainstream, ideological beliefs.

https://www.reddit.com/r/TheRedPill/

There's a documentary about one part of it: https://en.wikipedia.org/wiki/The_Red_Pill

Or you could look at the most extreme examples the other commenters are trying to present as the norm.

Oh boy, be prepared to rage. Just check r/redpill

Not easy watching, interrogation of a young man who took it to the end: https://www.youtube.com/watch?v=kGFWovUuWak