(covers information from several alternate timelines)
Aryans, according to Nazi ideology, were the superior race to which Germans belonged. As such, they hoped to rid the world of non-Aryans.
In an alternate timeline, extermination camps were a way of dealing with non-Aryans, though in 1944, Vosk suggested to a German general that they could create a deadly water-borne pathogen targeting non-Aryans. (ENT: "Storm Front")
In a World War II holoprogram in 2374, a German Kapitän told Karr that German blood was pure, and that "degenerate races" had brought corruption to their land, hence the need for Europe to be "purified". (VOY: "The Killing Game")
The term "Aryan" was used by scientists for describing Humans of European and West Asian heritage. This term was appropriated in the early 20th century by groups advancing a scientific invalid theory which stated Aryans were a superior race to other races of Humans. (Star Trek Encyclopedia, 4th ed., vol. 1, p. 47)