I will let the big time MMers answer this one, but it has to do with how many DIFFERENT projects you have run for X number of hours. So it's not just like I ran SETI or POGs for X hours, so I get X star. You have to run many different projects that long to equal that star. It's an interesting way to get people to crunch a lot of different projects. I'm obviously not that into it, but I think it's a pretty cool way of showing those who are dedicated to the broader spectrum of BOINC and not just a few pet projects.