A shrink sleeve is a thin tube
Shrink sleeves are thin, flexible plastic tubes (usually PVC with a blend of polyester) with the property that they shrink when heated. A shrinking stock is often used as an electrical insulation material for joints with uneven diameter: for example, a solder joint between two current wires. You can also use it well for keeping wire bundles together.
Applying shrink sleeve [YouTube] TlUeYP-LztA [/youtube]