Wax potting is basically adding wax to the pickup in order to get rid of the small air pockets that can potentially cause microphonic feedback at high gain levels. Pickups that aren't potted will have a very subtly different sound that can sound a bit more "acoustic" - they tend to be best for a clean type overall sound, eg jazz/blues. If you play high gain, you prob want potted pickups as unpotted might "squeal" at high gain levels.