For the uninitiated like me:

The manosphere has its own distinct jargon.[31] A central tenet of the manosphere is the concept of the red pill, a metaphor borrowed from the film The Matrix. It concerns awakening men to the supposed reality that men are the oppressed gender in a society dominated by feminism

( From https://en.wikipedia.org/wiki/Manosphere#Jargon, I landed there from https://en.wikipedia.org/wiki/Red_pill_(disambiguation) )