The abbreviation stands for "Red
Army". In USSR, it was
short for "Workers' and Peasants' Red Army".
The term comes from Russian civil war, 1917-1922, in which the victorious Reds (bolsheviki/ communists) fought against Whites (monarchists-
liberals) and Greens (anarchists). Red is a well-known political symbol of communism.
Today, most commonly, it refers for the land-based armed forces; may or may not include aviation; excludes navy and missile troops.