As an example, here are two two extremes of answers I've heard: "perhaps leading my own team, perhaps even starting my company" and "doing what I love: computers". The first one tells me that the candidate isn't interested in sticking around for long, the other tells me that this person will excel because they are naturally interested in computers and not just chasing the money. The second candidate is still here and doing really well, by the way.
The pattern I've noticed in response to this question is that those who mention something intrinsic related to the job, such as "doing what I love: computers" or "being the best programmer I can possibly be", usually do much, much better. On the other hand, the candidates chasing external rewards tend not to last long.