This is a 8mm Heat shrinkable tube. Heat shrink tubing (or, commonly, heat shrink) is a tube that shrinks in diameter when heated. shrinks to 50% of its diameter after heat is applied to it with the help of a heat gun.

heat-shrink sleeve containing a completely thermoplastic-covered interior surface.

On heating, the diameter of the sleeve reduces by 2:1, and the interior resin melts. On cooling, the sleeve and the resin harden. This combination guarantees resistance to pullout and perfect waterproofing for your connections, preventing a sleeve from slipping. In the case of a connector mounted on a cable (coaxial, for example), this sleeving ensures reinforcement at the place where the cable is generally subject to stress when the radius of the curve is low.