It's a 5mm 5 Meter Black Polyolefin Heat Shrink Tube Sleeve. Typically used to bundle wires and to seal electrical junctions, as well as to protect wire junctions from corrosion and abrasion. This 2mm Heat Shrink Sleeve is ideal for insulation. It features remarkable insulation properties, great stress control properties, long-term weather, and regular wear and tear resistance, ease of installation, and reliable performance in even the most extreme conditions.