[support] What is this error mean?

[A-NO-NE Music]

Drupal 6.13

I am doing a baby step to module world in order to hack into og  
detailed block.  So far is does work but comes with an error message:

"warning: Cannot modify header information - headers already sent by  
(output started at http://mysite.com/sites/all/modules/mymodule/mymodule.module:1 
) in http://mysite.com/includes/common.inc on line 141."

What does this mean and how do I remedy this?

Here is the code I have (which is pretty much copy/paste from what I  
found at the Drupal site except the first 2 lines).

<?php
// $Id$

/**
* Implementation of hook_og_links_alter().
*/
function mymodule_og_links_alter(&$links, $group_node) {
// drupal_set_message(''. var_export($links,TRUE) .'');
// Remove .. members link and change Invite link to Blablabla in group  
details block.
$links['invite'] = 'Blablabla';
unset ($links['subscribers']);
}


--
- Hiro

Hiroaki Honshuku, A-NO-NE Music, Greater Boston
http://a-no-ne.com   http://anonemusic.com