Via Wikipedia:

The five-pointed red star is a symbol of communism as well as broader socialism in general. The red star was used a revolutionary symbol after the October Revolution and following civil war in Russia. It was widely used by anti-fascist resistance parties and underground organizations in Europe leading up to and during the Second World War. During the war, the red star was prominently used as a symbol of the Workers’ and Peasants’ Red Army from the Soviet Union, which liberated its country from the invading forces of Nazi Germany and went on to rid the rest of Eastern Europe from the fascist occupation forces, achieving absolute victory and ending the war at the Battle of Berlin. In what was later dubbed the “Eastern Bloc,” fascist dictatorships overthrown by the Red Army invasion were replaced by socialist states that were politically loyal to the Soviet Union, while countries in Western Europe remained economically dominated by the monetary institutions of the capitalist world. Most states in the Eastern Bloc incorporated the red star into state symbols to signify their socialist nature.