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.
Not that many shapes are easily fashioned for producing crude flags. The Chinese flag represents the party with the big star and the four revolutionary (in the semi-feudal period) classes of the peasantry, proletariat, petty and national bourgeoisies.
Yellow has been a common secondary color for communist iconography and the star was an important regimental symbol. The two overlapped over time.