What is antifa and why is Donald Trump targeting it?

Donald Trump promised on Twitter this week that “the United States of America will be designating ANTIFA as a Terrorist Organization”. Legal experts immediately threw cold water on the proposal, arguing that there may be no means by which any domestic entity can be designated in this way....
https://www.theguardian.com/world/2020/jun/06/what-is-antifa-trump-terrorist-designation