The mode of production is the system under which goods and services are produced. According to Marxism different modes of production are Feudalism, Capitalism, Socialism, and Communism. Other people aqnalyse things differently.
According to Marxism in a socialist revolution, the capitalist system would first be overthrown, and replaced with socialism. A transitional system, the Dictatorship of the Proletariat, would take place. Eventually, communism would be achieved, and the state would be abolished. Again other people aqnalyse things differently.
The USSR, and other "communist" states, were never communist. They were, at least nominally, socialist states aspiring towards communism.