Your interviewer is asking this question to get a feel for your supervisory experience and see if you would be a potential hire who could lead and mentor others on the team. Be very open and honest about any leadership experiences you have and describe what that experienced entailed to your interviewer. Prior to your interview, be sure to put some thought into your future with this organization and if you'd be comfortable in any mentoring or supervising duties with them. Keep in mind that this isn't a make-or-break question, especially if the position you are interviewing for doesn't have a supervisory-type job title. Your interviewer is simply looking to hear about your interests down the road, so be honest with yourself as you answer.