Seems like a silly question but I couldn't find any satisfactory explanation online. Five finger hand? Five continents? Stylish hat worn by Che Guevara? It's in basically every revolutionary flag and even in some social-democratic parties, so it must mean something.
My knowledge on the star in Vietnam at least is each point represents a group of people: proletariat, peasantry, military, intellectuals, and petty bourgeoisie.