it has 96000 miles. the check engine light comes on when the engine shuts off but is not on during normal driving

i've the term pulling codes thrown around here. could someone explain what this means? I can really get this done at autozone for free?

2000 Contour SVT in Silver Frost #1633 of 2150 -17inch deep dish chrome rims -driller racing rotors -modified intake with open-air K&N filter ...TruBenz exhaust is next!