For me personally, the classes that really helped me were General Psychology, Biological Psychology, and Cognitive Psychology (or some sort of Learning/Memory course). I was a psychology major however so these courses were part of my curriculum and understandably not everyone has to/will take these or similar courses. If possible I would recommend taking at least a General Psychology course not only for the MCAT but some Med Schools recommend/require a semester of a social science (could also take Sociology). The rest can honestly just be memorized as terms and/or might be encountered in a Biology course (anything about neurons and the nervous system).