A Double D-Ring buckle uses two metal rings that you thread the strap through to lock it in place. It's the same system used on motorcycle helmets, and it's extremely secure — it won't accidentally come undone in a crash.
That said, Double D-Rings are slow to fasten and unfasten, and they're frustrating to operate with gloves on. That's why you rarely see them on recreational ski helmets. They're mostly found on some racing helmets where maximum security matters more than convenience.
For most skiers and snowboarders, a Fidlock magnetic buckle or even a standard side-release buckle is a much better choice. Both keep your helmet secure but are far easier to use on the mountain. Unless you're competing in ski racing, there's no real reason to choose a D-ring helmet.