[Serious] Can someone explain what a "tankie" is?

I'm very OOTL when it comes to a lot of the major divides in online communities because thankfully I've managed to avoid most of the arguing. I've noticed a lot of instances say "no tankies" as one of their first rules and figured this might be the only place I could get a real answer.

Was there something specific they did to piss off everyone? If I had to take a shot in the dark, are they just people who are really pro-military?