I got a player who had 5 star potential and a 5 star rating, because he's played to his full potential does that mean he's no good.
No, it means he is a very good player with a very good long term potential, meaning this is probably a player you want to draft, depending of course on your needs and how you plan to run your team, finances, transfers, etc.
Also if a player has a higher potential than his star rating does that mean they still have something in the locker
Higher potential means he will have a higher cap on his training and in theory be a better player in the long run (if trained properly) when compared to another player with the same star rating but with a lower potential.