With this question the interviewer is trying to ascertain how knowledagble you are regarding stress management and how well you apply those techniques to yourself and others.  In your answer, detail some constructive ways in which you handle stress.  You might mention reflecting at the end of a stressful day on what went wrong and how you can do better.  Tell the interviewer that you often devise plans on how to deal with situations that are stress-inducing for you or your team.