I believe its a new term for what used to be called the radical left. There is evidence they go beyond attacking nazi's, to attacking journalists or just people who "look" like nazis.

Antifa is a good example with their huge riots and firebombing of cars during Trump's inauguration.

The son of Israel's PM considers them a bigger threat to the jewish people than actual neo-nazis (surprising to me). http://www.timesofisrael.com/netanyahu-junior-says-leftists-...