The "WWW-Authenticate" header is part of the HTTP protocol used by web servers to request authentication from clients. When a server requires a user to log in, it sends this header along with a 401 Unauthorized response. This informs the client that authentication is needed to access the requested resource. The header specifies the authentication method, such as Basic or Bearer, and may include additional parameters. Clients, like web browsers or applications, use this information to prompt users for credentials, which are then sent back to the server for verification.