Doc's question strikes at the heart of what Ray is doing and the lives that Ray touches as he does it.  When Doc Graham asks the question to Ray, it is at the point where they are talking with one another and Ray is trying to figure out the how and when of what time travel has done with him:

I had this feeling, in there when I woke up- a pulling, like there was a magnet somewhere drawing me slowly toward it.  Are you a magnet, Ray Kinsella?

Doc's construction of the question is significant.  A magnet is something that pulls items together, bringing that which was separate as close together as possible.  Ray is a magnet of sorts in how he brings people like Shoeless Joe, Salinger, and now Doc Graham together with what they once lost and loved.  The question strikes at the essence of Ray's being.  He is a "magnet" of sorts as he sees what people have been separated from and makes them whole. The greatest significance this has is in his own life in terms of how Ray is brought closer together with his dreams and his past.  Doc's question is significant because it helps to capture the essence of what Ray is and what his purpose is in both the novel and the world.