Per Wikipedia, “Tankie is a pejorative label for communists and those who align with Marxism–Leninism ideology.” That’s basically what you get when you ask people to define, “tankie.”

But, as with most perjoratives, its usage has expanded. It can still be used in its original meaning but it’s often used much more broadly. If you do a search on how people use the word “tankie” (like in comment threads) you’ll see it’s now commonly used to describe anyone who isn’t sufficiently critical of China and Russia and sometimes as a modern synonym for “un-American”.