This means there is an output where there should not be one, an output sounds technical but it is usually a single blank space where it should not be!

 

This is either at the very start of the script so it may look like

 

  <?php 

when it should look like

<?php

 

Or at the end

 

  ?>

when it should be

?>

 

The file name with the error will be given in the message you recieive.i.e

 

output started at /home/xxxxxxx/public_html/catalog/includes/application_top.php

 

 

The error happens when a file is worked on by add in code, a contribution or similar.