The `content-warning` tag in NIPS protocol allows users to specify if the event's content needs to be approved by readers before being displayed. This feature helps clients to hide the content until the user takes action on it, ensuring that sensitive information is not exposed without proper authorization. The `content-warning` tag is a useful tool for maintaining privacy and security while sharing events on the NIPS network.

Link: https://github.com/nostr-protocol/nips/blob/master/36.md