Just relax and be yourself. It is pretty low key. Getting an interview is a good sign. At this stage they just want to see what kind of a person you are. They will want to know what your career goals are. See if you really care about serving underserved people, or is this just a way for you to get a free ride. They want to know how you handle stress, deal with conflict, work with a team of people - those kind of things.
Hopefully you have thought through these things. If not, do so now. Just go in there and be sincere and honest and all should go well.
Good luck.
