Okay, lemme dispel the "antifa is just short for anti-fascist" narrative that's going around Twitter right now.
"Antifa" is short for "Anti-Fascist Action," a loosely-organized group of cells who hold revolutionary socialist beliefs and demonstrate accordingly.

Most antifa cells believe that the true way to fight fascism is to fight capitalism and statism—the two elements they believe will inevitably facilitate fascism. They believe that an international popular uprising is necessary to accomplish this, to truly defeat fascism.

This uprising necessitates violence against individuals and state forces. Antifa believes that violence is necessary because they reject liberal notions of free speech being able to defeat fascism. They also believe that this violence is a form of self-defense against fascism.

Just being anti-fascist does not make you antifa. Most people, when asked, are against fascism. Being antifa requires more, namely, recognizing that fascism and capitalism are linked, that capitalism is the backbone of oppression, and that direct action is necessary for change.

Anyway, if you want a resource for this, I do recommend reading Mark Bray's "Antifa: The Anti-Fascist Handbook." It's an important read about the history of antifa groups worldwide, and does a good job of relating what they believe and why.