A sacrificial layer is a disposable layer, in this case made of silicone, that is put around a carabiner to protect your rigging from wear. Sacrificial layers help to keep your rigging safe by sacrificing themselves before damage is done to aluminium or nylon components.
If a steel rigging component is connected to an aluminium or fabric one the movement in the system can cause the aluminium or fabric to wear away as it's not as hard as the steel. This wear can weaken the swivel/figure 8/sling and make your rigging less strong and therefore unsafe. This can be avoided by putting this layer of silicone tubing around one of the components. The silicone is pretty durable but will start to wear long before the aluminium or nylon is damaged.
The sacrificial layer should be checked before each use. If worn through, the silicone sacrificial layer should be changed immediately. You should also check the carabiners and other components to make sure they remain undamaged. For more information about sacrificial layers read this article.
The silicone tubing on this page is sold per yard. Its inside diameter is 0.6" (15mm) and its outside diameter is 0.8" (20mm). The tubing is easy to cut with a sharp knife or pair of scissors and one yard will provide many sacrificial layers.