The nozzle cover for the Ultimaker S5 and S3 shields the print cores from cold airflow from the fans, which helps the print cores to maintain a stable temperature while printing. The cover also helps to prevent backflow of material into the print head when something goes wrong during printing.
The heat of the nozzles may cause wear on the nozzle cover over time. It is recommended to check the quality of the cover at least once every three months.
For instructions on replacing your Silicon Cover on your Ultimaker S5 or S3 goto.