1. slang The choice to become aware of and engage with true reality, despite the pain or hardships that it may entail. The term contrasts with "blue pill," the choice to remain willfully ignorant of and disengaged from the real world. The imagery is a reference to the 1999 film The Matrix, in which the main character is literally given a choice between a red pill, which will allow him to experience true reality for the first time, and a blue pill, which will result in him to continuing to live as a prisoner in a simulated world. Taking that philosophy course was like choosing the red pill—I feel like my eyes have really been opened.
2. slang A man who holds and promotes a conception of dominant masculinity rooted in misogyny and opposition to feminist ideals. The term contrasts with "blue pill," a derogatory term for a man who remains oblivious to or rejects such a conception of masculinity. (The origin of such imagery is detailed in Definition 1.) "The Red Pill" is the name of a prominent subcommunity on the online forum Reddit that features discussion of such topics, typically strategies for sexual conquest. He sees himself as a red pill, but he's really just repeating all the things he reads online about how to be an alpha male.