These are silicone socks for your E3D Revo-Hotend.
The socks keep your heating block clean and free of burned and blackened plastic. They also keep the heat in the hot end and away from your print.
These socks are made of a heat-resistant, non-stick silicone material that prevents the formation of melted plastic. This helps with sticky materials like co-polyesters and filled composite materials that like to build up on nozzles.
Silicone socks also give a huge increase in temperature stability, especially if you use a cooler with a strong airflow around the nozzle to cool the print. The additional cooling combined with the reduced heat radiation from the block to the print improves the ability to print large overhangs and bridges, while the hotend temperature remains extremely stable.
