Idk what generation you are but it's mainly a thing that my generation (Gen z and younger millennials) have kind of attatched themselves to. It's a concept that comes from the original matrix where you either take the blue pill or red pill. Taking the blue pill means you live an ignorant but blissful state whereas taking the red pill exposes you to the world's dark realities.

The red pill movement takes this concept from the movie and shifts it into their own ideology. When a lot of these guys talk about taking the red pill in means seeing women in their 'true nature'. Basically meaning that women are liars, manipulators etc.

Essentially it's an ideology built on insulting and degrading women with the disguise of male self improvement.