This variable is used to specify if you want the HTTP header [1] to only contain the data relating to the SPIP version and the plugins employed, which SPIP does by default.
Here is an example of a screenshot provided by Firebug :
To restrict the data contained in the HTTP header as much as possible, just add the following line to your config/mes_options.php file (see dedicated article):
$GLOBALS['spip_header_silencieux'] = 1;
