Template prefilters are PHP functions that your templates are ran through before they are compiled. This is good for preprocessing your templates to remove unwanted comments, keeping an eye on what people are putting in their templates, etc.
Smarty will pass the template source code as the first argument, and expect the function to return the resulting template source code.
Example 218. Using a template prefilter
This will remove all the html comments in the template source.
// put this in your application
function remove_dw_comments($tpl_source, $smarty)
// register the prefilter