Like most words it can mean different things depending on context. I’ll do my best to cover a few without spoiling it with my own opinions.

The most common usage is as a blanket pejorative aimed at anyone who identifies as leftist but also openly endorses authoritarian means or ends.

There are also those who embrace the term and they are also not all the same. There are Marxist-Leninists who believe the only path to a stateless egalitarian society is through a revolutionary vanguard party. There are also those who argue that egalitarian society can only be achieved and maintained through benevolent authoritarianism.

In any case, the term carries an implication of authoritarianism and/or revolutionary violence, hence “tanks.”