It means that his skills have taken up 45-48% of the potential he has. I'm sure you know that the higher the potential, the more room there is for more skills. for example, if a player with allstar potential has a build of 14/9/14/10/10/10, he has used up 88-97% of his potential. If that same player had perennial allstar potential, then he has used up 80-88% of his potential.