I thought I read somewhere that the star system was meant to indicate how likely a recruit would be able to contribute as a freshman.
Each site has variances on what stars mean. Here's the excerpts from ESPN's football recruiting homepage.
http://sports.espn.go.com/ncaa/recruiting/football/news/story?id=6635735
5* Rare prospects: 100-90
These players demonstrate rare abilities and can create mismatches that have an obvious impact on the game. These players have all the skills to take over a game and could make a possible impact as true freshmen. They should also push for All-America honors with the potential to have a three-and-out college career with early entry into the NFL draft.
4* Outstanding prospects: 89-80
These players have the ability to create mismatches versus most opponents and have dominant performances. These players could contribute as a true freshmen and could end up as all-conference or All-America candidates during their college careers and develop into difference-makers over time.
3* Good prospects: 79-70
These players show flashes of dominance, but not on a consistent basis -- especially when matched up against the top players in the country. Players closer to a 79 rating possess BCS-caliber ability and the potential to be a quality starter or all-conference player. Players closer to a 70 rating are likely non-BCS conference caliber prospects.
2* Solid prospects: 69-60
These players are overmatched versus the better players in the nation. Their weaknesses will be exposed against top competition, but have the ability to develop into solid contributors at the non-BCS FBS level and could be a quality fit for the FCS level of play.
1* Prospects: 59-50
Players have some redeeming qualities but are not projected to contribute at the FBS or FCS levels.
Prospects: Not ranked or NR
Evaluations are pending film evaluation. These players will have a grade of "NR" and that means we have not had a chance to fully evaluate the prospect.
Rankings prior to the 2013 class
Players in the classes before 2013 were ranked on a different scale with five-star prospects falling within 85-100, four stars between 79.5-84.99, three stars between 75-79.49, two stars between 68-74.99 and one stars between 55-67.99.