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.