It is a derogatory term, so it's bound to get overused a bit, but it generally refers to "leftists" who like the USSR and simp for existing authoritarian states with "communist" parties, like the PRC, Cuba, Vietnam, NK, and even Russia despite it having nothing to do with any kind of socialism anymore. Some of them are indeed war crime enjoyers, while others try to dress it in denial and propaganda.
As a counter example, socdems, democratic socialists, anarchists are not tankies, it's not just anyone on the left spectrum.