See: https://theanarchistlibrary.org/library/ziq-tankies-and-the-left-unity-scam

But if that's far too much reading - here's a TL;DR that's two paragraphs from it that sums it up nicely.

Tankies celebrate Lenin and Trotsky's massacres of socialist revolutionaries, including the Mensheviks, the sailors of Petrograd, the Socialist Revolutionaries, the anarchists, unaffiliated peasants who had their food confiscated and so on. Tankies also celebrate murdering 'kulaks', a word they use to describe any peasant that resisted Soviet imperialism, but especially the Ukrainian peasants that resisted sending all their food to Russia, which they rightly guessed would lead to mass-starvation and one of the worst atrocities in history; the catastrophic Holodomor man-made famine.

A tankie is anyone who presents themselves as a 'communist', but engages in apologism for torture, slavery, imprisonment, imperialism, capitalism, genocide and the erasure of actually liberatory movements. A tankie is anyone that claims communism can be achieved by replacing a state with another state. A tankie is anyone that will swear up and down that state-capitalism, dictator personality cults and ecosystem-destroying mass-industry will eventually lead to communism through the "withering away" of the brutal state that they uphold.