If you're asking about the origin of the name Dan, it's a short form of the name Daniel, which comes from the Hebrew name "Dani'el", meaning "God is my judge".
It's possible you've heard people use "Dan" in a way that you don't understand, but it's more likely that it's just a nickname or a familiar way of referring to someone named Daniel.
If you can give me some context about where you heard "Dan" used, maybe I can help you understand what it means in that specific situation.