Proper use of the_content filtering

You want to add a filter to the filter queue on the_content:

add_filter('the_content', 'anything');

WordPress core then calls apply_filters within the_content function to run all filters in the queue:

function the_content( $more_link_text = null, $strip_teaser = false) {
    $content = get_the_content( $more_link_text, $strip_teaser );
    $content = apply_filters( 'the_content', $content );
    $content = str_replace( ']]>', ']]>', $content );
    echo $content;
}