Technically the term barbarian came from Rome and it was a derogatory term for anybody not from Rome. In dungeons and dragons 5e we mostly get the term from "Conan the Barbarian '', a musclebound survivor of savage wastelands dressed in a leather loincloth and madly hacking away at evil cultists with a massive axe. So think of a barbarian as character that can survive almost anything as they murder everything around them in a bloody frenzy.