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.
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.