When this question is posed to you the interviewer is looking to gain information regarding whether you are able to balance your professional responsibilities along with your personal life. Keep in mind that balance can be crucial, especially if the position in question has a lot of responsibility. Employees that are not able to achieve balance in their professional and personal lives will likely find themselves running into problems. They may work too hard and become burned out or see their personal lives suffer. On the other hand, they may neglect their professional duties. The key is to achieve a balance between the two and you need to be able to convey to the interviewer that you understand the importance of this and how to accomplish it. Describing your philosophy regarding your professional and personal lives will help you to do that. For example, you might describe some specific activity that gives you a break from work, ensuring that you are able to return refreshed and then perform at optimum efficiency.
Your answer need not be lengthy and you certainly do not need to go into all of the activities that you enjoy in your spare time but it is important for you to convey to the interviewer that you have a life that is well-rounded and that your life is not one-dimensional in terms of either work or personal pursuits.
The interviewer may also be looking for an indication that your personal interests are compatible with your professional goals. You can demonstrate that there is common ground by relating some leisure activity which you enjoy that is related to the duties of the job description. For example, if you enjoy a particular sport you might mention that in your answer and relate how the pacing and strategy involved in that sport is indicative of your professional approach as well.
Once again, remember that your answer does not need to be lengthy. The main goal is to make sure that you convey to the interviewer the common ground shared between your personal interests and your career goals and also that you understand how to remain refreshed during your off time so that you can give 100% when you are at work. If you are able to do this you should be able to ace this interview question.