The following link led me to this from Rivals:
The ranking system ranks prospects on a numerical scale from 6.1-4.9.
6.1 Franchise Player; considered one of the elite prospects in the country, generally among the nation's top 25 players overall; deemed to have excellent pro potential; high-major prospect
6.0-5.8 All-American Candidate; high-major prospect; considered one of the nation's top 300 prospects; deemed to have pro potential and ability to make an impact on college team
5.7-5.5 All-Region Selection; considered among the region's top prospects and among the top 750 or so prospects in the country; high-to-mid-major prospect; deemed to have pro potential and ability to make an impact on college team
5.4-5.0 Division I prospect; considered a mid-major prospect; deemed to have limited pro potential but definite Division I prospect; may be more of a role player
4.9 Sleeper; no Rivals.com expert knew much, if anything, about this player; a prospect that only a college coach really knew about
This question arose in the thread about Demetrius Hart and his potential to be a 5-star. As someone else mentioned, Michigan fans thought Marvin Robinson and Ricardo Miller were going to be 5-stars about a year ago, so don't let your expectations get too high.
A 6.1 rating is given to all 5-stars. As it says, 5-star players have "excellent pro potential." This is a problem for diminutive backs and slot receiver types, whose potential is limited by the types of offenses run and the other athletes in the NFL. That's why you'll see very few 5'8" players being given 5-star rankings by Rivals - because there aren't many 5'8" players in the NFL.