it seems that "tankies" are very militant communists.
In the original sense of the word, it was specific to people who favored Soviet military intervention in Eastern Bloc countries. Today, I see it used on the UK political subreddits more-commonly used more broadly for people who are really anti-West and authoritarian far-left. Like, if someone takes the position that Stalin had the right idea and his vision of the Soviet Union was the right way for the world to go, they'd probably be called a tankie regardless of their specific views on the Soviet interventions in Eastern Europe.
It isn't used much here in the US, that I'm aware of.