A fan can be a lot of things. You can sell software development services, and your fan may be a graphic designer who uses you every time to build out the back-

A fan can be a lot of things. You can sell software development services, and your fan may be a graphic designer who uses you every time to build out the back-end for the beautiful designs they make. You do a good job, you're their go-to person, they are your "fan".