Credits are like tokens at a game house. Certain games (cars) use certain number of tokens (credits) in order to use them. You get a certain number of credits and have program sets you can buy. The number that come with a stock HPTuner are fine for getting just two years unlocked on a GP, or other V6 car.
My main qualm, is your paying $100 more for it than a PowerTuner, and the PowerTuner comes with an unlocked year range from '98-02.
_________________________
'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne