E3d supplied hot end sock.
Pro silicone socks are made to insulate the heater block and nozzle for more efficient, stable thermal performance. The pro sock goes a step further than the standard sock, covering the majority of the nozzle, leaving just the tip exposed. This makes it ideal for small nozzles sizes (0.15 and 0.25mm).
Made from an inherently non-stick high-temperature silicone, the pro sock helps keep your nozzles and block free from plastic residue.
These socks are sold individually. They are consumables, you ought to grab a few.
These socks are E3D Blue.