This feature is designed to delegate user authentication to third-party systems (proxies/servers).
When this feature is activated, SonarQube expects that the authentication is handled prior any query reaching the server.
The tool that handles the authentication should:
intercept calls to the SonarQube server
take care of the authentication
update the HTTP request header with the relevant SonarQube user information
re-route the request to SonarQube with the appropriate header information
All the parameters required to activate and configure this feature are available in SonarQube server configuration file (in $SONARQUBE_HOME/conf/sonar.properties).
Using Http header authentication is an easy way to make your SonarQube deployment able to sustain inhouse SSO implementation.