Coding ring for Neutrik connectors. This little plastic ring is the perfect tool to mark a cable so that you can identify it among other cables.
It can't fall off or move along the cable, since it is required to unscrew the connector barrel to put it on, replacing the stock one that is usually black. It is just a plastic ring, there is no glue or anything that could leave residue, once put it on it feels like part of the connector.
The color is a little darker than the picture, but it is still clearly visible.