What does this meen

I have alleady change in htdocs http://localhost
this warning i see on my webpage, what does this meens.

Warning: Cannot modify header information - headers already sent by (output started at C:\Documents and Settings\Timsa\Mijn documenten\xampp\htdocs\phpAlbum\config.php:18) in C:\Documents and Settings\Timsa\Mijn documenten\xampp\htdocs\phpAlbum\main.php on line 224