Well, others have answered you that it's the symbol of "worker and peasant Red Army", as hammer and sickle symbolize. And a five ended star was, I think, a military symbol of limited popularity in Russia before Communism, while Red Army simply made the color constant. The star was also initially upside down, as a way to defy Christianity, this is not a joke. But later they, apparently, decided that it being upside down is juvenile.