Usually people who advocate military communism, such as defending tiananmen square, Stalin, and more. Basically it's usually the belief that anything a government professing to be communist does must be good.
I should emphasize that a lot of pro-Western outlets and commentators have recently weaponised the term to discredit any diverging points of view re-the Ukraine War. So someone like Cornel West would be a "tankie" by this point of view, which is actually kind of disgusting in how dishonest it is.