Many people would answer a question like this with some information about their education and credentials, and while this is acceptable, it should be followed up with a bit of more personal information. You should take some time before your interview to reflect upon this so that you can provide your interviewer with an answer that is meaningful. For instance, you can provide some information about a particularly difficult case, whether you actively participated in the case or observed as part of your training. Similarly, you could provide information about a personal experience, particularly if you or one of your own children was helped by a guidance counselor in the past.