The interviewer is really looking for past experiences that you have been in that have been stressful. They want to get to know your background better. Highlight these examples, tell them how you remained calm, and held your composure. Tell the interviewer what you learned from those experiences. A good example, when you have been in stressful situations, you have found its best to remain calm so you can think through any challenges that you are facing. Try to think of these examples before the interview, because this is a very common interview question and may likely be asked.