AT = alpine touring. They are bindings that allow you to release your heal, allowing you to essentially turn your downhill skis into 'cross country' style skis. Combined with skins (carpet like things that go on the base of your ski - making your skis grip on snow instead of slide) you have the ability to walk uphill with your skis.
They aren't as good in bounds, but some are better than others. If you aren't gonna be out in the backcountry climbing up slopes, they aren't very useful.