Short answer: folk who are so pro-Communist that they think the Tiananmen Square massacre was a good thing. The massacre in which the CCP bulldozed protestors with tanks.
Hence, "tankie"
Edit: apparently I'm wrong on the origin of the term, and it predates Tiananmen Square. See the reply below.
I will add that given the current projection of power currently, China is a far more powerful authoritarian communiststate than Russia, so most tankie propaganda is China-centric these days.
Yeah, extreme left wing basically. Think that capitalism should be overthrown by force. Same as jan6-ers but pro Communist