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.