It's basically a test you take in high school and you can get a score from 1-5. If you get at least a 3 (some colleges require at least a 4), you get college credit for that class (eg if you get at least a 3 in calculus BC you can skip calculus 1 and 2 in college).
This is why don't generally do AP classes, I'd rather do dual credit and ensure I get the credit, and don't have to take the class in college. Also from my personal experience DC classes are easier than AP, also a bit of advice don't take AP Lang and Composition because that class sucks