Lemmy was created by tankies, so people are rightly going to have things to say about them here.

I was in the same position. I looked into it a little bit and my TL;DR version is that tankies are people who in general are apologists for the Russian and Chinese governments using fascistic and dictatorial measures. I'm guessing that more broadly speaking this could apply to supporting any government that uses violence against its own people, but it usually comes up in the context of those two countries.