What do the little stars on USMC military ribbons stand for?

The stars denote that the recipient has received the award more than once. For example, if a Marine has a Navy Achievement Medal/Ribbon with 2 bronze stars on it, the Marine has been awarded the Navy Achivement Medal 3 times. (The first would be represented by the initial medal/ribbon, the second would be the ribbon with one star, the third would be the ribbon with two stars.)