system
A chat on subreddit /r/NoStupidQuestions
human
Antifa is an extremely polarized concept because it seems that far-left and far-right people fundamentally disagree as to what "antifa" is. I will try to give perspectives from both sides based on common arguments I see.

From the conservative perspective, Antifa is a group (but more of an ideology) that claims to be anti-fascist while engaging in fascist activities akin to the brown-shirts in 1930's Germany who were pretty much the footsoldiers for the early Nazi party. These activities tend to be anti free speech and condemn people, sometimes with physical violence, for saying things - or promoting policies - that antifa broadly deems "fascist." Conservatives point to the 2020 riots across the U.S. as examples of antifa's destructive power.

From a liberal perspective, Antifa is much more of an idea that is brought together under an umbrella of localized small groups that are actually anti-fascist. Some of these small groups might be more radical than others, but the majority of them promote peaceful opposition to radical right-wing ideas. Thus, someone wearing an antifa shirt is simply promoting an ideology that is actually, in good faith, anti-fascist.
gpt
The Nazi comparison is really important and I wish your comment stressed it more.

While many conservatives do genuinely see Antifa as "the real Nazis", the name, aesthetic, and protest methods of modern Antifa groups come from Antifaschistche Aktion, a militant resistance group within Nazi Germany.  Because Antifa today is not a formal organization, there's not a direct continuity with AA, though the original was still quite the inspiration for modern Antifa.

It's also worth noting that the moral and philosophical justifications for actions taken by modern Antifa are based strongly in ideas that came from survivors and scholars of the Holocaust, including the Paradox of Tolerance.