Back panel access means the main compartment opens from the side that sits against your back—the pack hinges open away from you like a book. This design is especially handy for backcountry touring because you can get into your gear without removing your skis from the front carry system. Just slip the pack off your shoulders, unzip the back panel, and grab what you need.
Another bonus: since the opening faces your body when closed, it creates an excellent weather seal. Snow and moisture have a hard time working their way in, which is great on stormy days or multi-day trips.
The main trade-off is that you must take the pack completely off to access anything—there's no reaching in while wearing it like you can with a side-access zipper. But if you frequently carry skis or a snowboard on your pack and want quick, full access without disturbing those attachments, back panel access is tough to beat.