Echo term slug op post on archive page

The get_the_terms function returns array of WP_Term objects.
So you need to use something like this to echo a single term slug:

echo $terms[0]->slug;

Also be aware of results of this function. As documentation says it returns:

Array of WP_Term objects on success, false if there are no terms or the post does not exist, WP_Error on failure.

So you need some checks before trying to echo terms. The following code may help you.

$terms = get_the_terms( get_the_ID(), 'type_kennisbank' );

if ( $terms && ! is_wp_error( $terms ) ) {
    foreach ( $terms as $term ) {
        echo $term->slug;
    }
}