This is big one! I generally start with the distinction between structural problems and behavioral problems — e.g. did someone not go to the gym because it was two hours from their house or because an alarm induced a temporarily hampering psychological state? Most audiences seem to find that at least somewhat useful. Expanding into what a behavioral problem is, I then (usually) say that there are all sorts of contexts that elicit psychological phenomena that prevent people from turning intentions into actions– I think most of  BSci falls somewhere in there. It’s always good to not assume people know specific examples (I don’t know Dan Ariely’s vacation one, e.g.) … better to give a concise explanation of the setup just in case.