It's an informal 'quite' as an adverb. It's also used as a noun meaning some certain level someone or something has reached. Sometimes it sounds a bit playful because of its tone, for example: A: Hey, I fixed the bed lamp. (Turns it on) B: 오, 제법인데. (이런 것도 할 줄 아네? You knew how to do something like this. I didn't think you would.) A: (singing so well) B: 노래 제법 하네? You sing quite well. (I didn't think you would.)