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.
It's a word liberals use to dismiss anyone to the left of them. in the same way Maga dipshits say 'woke' about liberals. Just an amorphous slur that means let's stop talking about issues that make us question the dominant narrative.
[Aaaand here come the downvotes which will prove my point]
I guess follow your beliefs on this one. Also, it's possible that by you being there, you add to the number of active users on that instance, therefore increasing the appeal for newer users to join in and possibly donate to the founders. (which are tankies)