Skivers are tools used to remove thin layers of leather material. They generally have a very sharp cutting blade. When the blade is drawn against the leather grain with some pressure applied down upon it, the blade cuts through the leather as it moves along, shaving off a thin layer.
Skiving is helpful In many facets of leather work. It is used, for example when making belts. At the point where the leather is folded over to secure the buckle, there would be double the leather thickness since the material is folded over. This could be bulkier than necessary, as well as potentially uncomfortable to wear. Skiving can be used to thin out the leather, on both sides around the fold, so that when they are joined together it is only about one layer thick. This will look cleaner and wear more comfortably.
This same process can be applied to bag straps, saddlery, and any leather work that involves folding and joining thinker pieces of leather. Skivers are available in hand-held versions, manually drawn against the leather. Skivers are also available in tabletop versions, where the leather is pulled through the skiver.
Additionally, there are skipping knives, used to free-hand skive. Carefully removing leather layers is part art and part skill. Most leather craftsmen will at some point have one or several skivers on their leather craft tools list.