Mark Bray explains the history, goals, and philosophy of antifa in Teen Vogue. Every paragraph is a jewel, you should go read it. Just a taste:
Antifa grows out of a larger revolutionary politics that aspires toward creating a better world, but the primary motivation is to stop racists from organizing; doing that can take many forms, and so the tactical repertoire of anti-fascists is broad.
The vast majority of what they do does not entail any physical confrontation. They focus on researching white supremacists and neo-Nazis across different social media platforms, figuring out who their leaders are, what other groups they are networking with, [and] where they are trying to hold events, so they can contact hotels or local venues to get the owners to cancel the events and, if they refuse to cancel, organize a boycott or campaigns of public pressure against them. They also organize public education campaigns and form alliances with unions and social movements to organize large demonstrations. Part of it, however, and this is what gets the most attention, entails self-defense and, at times, confronting these groups before they can gain enough momentum to promote their politics.