These are silicone socks for a Phaetus Rapido HF or UHF hot end.

The socks keep your heating block clean and free from burnt and blackened plastic. They also keep the heat in the hotend and away from your print.

These socks are made of a heat-resistant, non-stick silicone material that prevents melted plastic from forming. This helps with sticky materials like co-polyesters and filled composites that like to build up on nozzles.

Silicone socks also achieve a huge increase in temperature stability, especially if you use a cooler with strong airflow around the nozzle to cool the print. The extra cooling combined with reduced heat radiation from the block to the print improves the ability to print large overhangs and bridges while keeping the hotend temperature extremely stable.