This action hook is used to add additional headers to the outgoing HTTP response.
As an example: HTML5 Boilerplate provides an X-UA-Compatible meta element by default. This element breaks validation, but can be moved to a header. Adding the following to functions.php fixes the validation issue and provides IE users a better experience.
add_action( 'send_headers', 'add_header_xua' );
function add_header_xua() {
header( 'X-UA-Compatible: IE=edge,chrome=1' );
}
The 'send_headers' hook is found in /wp-includes/class-wp.php, within the send_headers() method of the WP() class.