when use function the_content break

by

Because you only return $content from inside your if – you should always return it:

function beschreibung_kriterien( $content ) {
    if ( is_page_template( 'template-test.php' ) ) {
         $content .= '<h2>Test</h2>';
    }

    return $content;
}

add_filter( 'the_content', 'beschreibung_kriterien', 5 );

This just goes to the show the importance of proper indentation!