When snow is wet it can start to create a vacuum under the ski which stops the skis from sliding so easily, as can happen with slush. The worst snow for this though is generally fresh snow that still has the snow crystal structure but which is melting rapidly and starting to become wet very quickly. Because of the fine wet structure of the snow it moulds itself to the bottom of the ski very tightly making it very hard for air to get underneath. This creates a vacuum that makes it very difficult for the ski to slide. Skiing in this kind of snow can be very frustrating as skis can simply not slide across slopes that they normally would, and even skiing down steeper slopes can feel like someone has put the brakes on.