I’ll try to summarize it in a way that doesn't support or condemn it either way, since I think that sort of summary is most useful.

Red Pill is a way of behaving, and group of ideas, focused on the following:

Women all have the same desires, goals, and faults. Evidence otherwise is misleading and false and therefore must be interpreted through the Red Pill ideas.

Men can be divided into Alphas (sexually attractive men) and Betas (men who are “nice” and who women manipulate). There are other sorts of men, but these are the two types discussed most often.

Women cannot truly love men as whole people, and therefore should not be trusted.

Men, when interacting with women and trying to build relationships with them (whether as a one night stand or as a marriage), must keep this in mind, and behave as an Alpha, while letting go of the dream of finding a genuine, loving woman (since women cannot be genuinely loving).

There are a lot of overlapping beliefs that are common in Red Pill, Men’s Rights Activism, and Men Going Their Own Way, but these are the fundamental beliefs of The Red Pill in particular.

The Red Pill also has a large focus on certain aspects of self-improvement that will help a man to become more attractive to women, while also helping him to regard himself as high-quality even if any particular woman does not. This is the most common summary given to people who question The Red Pill.

The methods they advise for these things sometimes overlap with general wisdom on self-esteem, and at other times overlap more with the idea that self-esteem comes at least partly from lowering one’s opinion of others. This idea can be seen elsewhere even in as simple a situation as public speaking: people are advised to imagine the audience in their underwear so that the audience will seem silly. This style of increasing confidence is, in the Red Pill philosophy, generalized far beyond specific situations like public speaking..