comment_excerpt
comment_excerpt is a filter on a comment excerpt before it is used by the comment_excerpt() function. Note that the excerpt is passed through the get_comment_excerpt filter first, in the get_comment_excerpt() function, so it may be a good idea to use that filter, to cover all comment excerpt instances.
File: wp-includes/comment-functions.php
function comment_excerpt() {
echo apply_filters('comment_excerpt', get_comment_excerpt() );
}
This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('comment_excerpt', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.