It originally comes from the movie, The Matrix. Watch it if you're unfamiliar.
Blue pill = the reality that the creators of the matrix want you to see
Red pill = the reality outside of the matrix.
It means something different depending on who is using it though.
If you're a leftist/liberal type, red pill probably means something around being aware of systemic racism. They also use the term "woke" which means the same thing... I'm guessing they have some similar things for trans, gay, and other LGBT letters of the alphabet but I'm not in that crowd ever so I don't really know for sure.
If you're a red pill dating guy, red pill probably means that you think all women will leave you as soon as they find someone better (hypergamy)... or as they like to call it, "accept the reality of women." Blue pill is anyone who doesn't agree with your viewpoint or someone you want to attack because you're low self-esteem and it makes you feel good to pretend like you're "in the know" while others are idiots.
If you're in the MGTOW/manosphere, red pill probably means something about caring about men's rights... or if you're in the extreme of them, it means that you realize that the world is coming to an end, there's nothing you can do about it, and nobody cares about your rights because men are victims and there's nothing that can be done about it but cry and whine to other people on the internet about how unfair everything is because you're a man.
Black pill is nihilism... or specifically, thinking that the world is falling apart and it's inevitable.
My guess is there are other interpretations of red pill/blue pill depending on which part of the internet you're on. But that basically sums it up.
There are a bunch of other "pills" but I don't keep up with it because I think the more I keep up with it, the less braincells I'll ultimately have.