The interviewer would like to know about any formal training you have delivered, related to customer service. This training could include a robust onboarding plan for your new customer service reps, any on-the-job training you provide, courses you enroll your team in, or even books that you give to your staff to help them grow professionally.
Show the interviewer that you have an interest in bettering your customer service team, and yourself along the way. This question is a great time to ask the interviewer about their company-specific customer service training approach.