Wikipedia has a good definition:

Tankie is a pejorative label generally applied to communists who express support for one-party communist regimes that are associated with Marxism–Leninism, whether contemporary or historical.

History:

The term “tankie” was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain (CPGB) who followed the party line of the Communist Party of the Soviet Union (CPSU). Specifically, it was used to distinguish party members who spoke out in defense of the Soviet use of tanks to crush the Hungarian Revolution of 1956 and the 1968 Prague Spring uprising, or who more broadly adhered to pro-Soviet positions.