This is a Heat Shrink Tube of 5mm diameter at room temperature.
Heat shrink tube is a mechanically expanded extruded plastic tube ordinarily made of nylon or polyolefin, which shrinks when heated in an effort to return to the relaxed diameter it originally had when extruded.
Heat shrink is used to insulate wires, providing abrasion resistance and environmental protection for stranded and solid wire conductors, connections, joints and terminals in electrical engineering. It can also be used to repair the insulation on wires or to bundle them together, to protect wires or small parts from minor abrasion, and to create cable entry seals, offering environmental sealing protection.
The tubing is placed over the connection to be protected and then heated with an oven, hot air gun or similar tool. Convenient but less effective methods for shrinking the tube include a soldering iron (held in close proximity, but not touching the tube) or the heat from a lighter. These processes cause the tubing to contract as much as one sixth of its original diameter (depending on the heat shrink, a ratio of 2:1 is the most common), providing a snug fit over irregularly shaped joints. This provides good electrical insulation, protection from dust, solvents and other foreign materials, as well as strain relief. If overheated, heat shrink tubing can melt, scorch or catch fire like any other plastic.