Cold shrink tubing is an open ended rubber

Synthetic rubber

Synthetic rubber is is any type of artificial elastomer, invariably a polymer. An elastomer is a material with the mechanical property that it can undergo much more elastic deformation under stress than most materials and still return to its previous size without permanent deformation...

 sleeve, made primarily from rubber elastomers with high-performance physical properties, that has been factory expanded or pre-stretched, and assembled onto a supporting removable plastic core. Cold shrink tubing shrinks upon removal of the supporting core during the installation process and the electrician slides the tube over the cable to be terminated and unwinds the core, causing the tube to collapse down, or contract, in place.

Cold shrink tubing is used to insulate wires, connections, joints and terminals in electrical work. It can also be used to repair wires, bundle wires together, and to protect wires or small parts from minor abrasion.