Essential to climb without sliding back, "seal skins" are long strips of nylon or mohair that you can attach to the base of your skis or splitboard. In the direction of the fibres, you glide. Against the fibres, they grip to the snow and stop you sliding back. They fit on the tip and the tail and enable the skier/snowboarder to climb (they are then removed to ski downhill). Finding the right skins can be difficult, as there are several different design options and the size of skins can be complicated.
This guide explains the advantages and disadvantages of different types of skins and will give you an idea of sizing.