get_the_terms

 *
 * @since 2.8.0
 *
 * @param int $tag Optional. Tag ID. Defaults to the current tag ID.
 * @return string Tag description, if available.
 */
function tag_description( $tag = 0 ) {
	return term_description( $tag );
}

/**
 * Retrieves term description.
 *
 * @since 2.8.0
 * @since 4.9.2 The `$taxonomy` parameter was deprecated.
 *
 * @param int  $term       Optional. Term ID. Defaults to the current term ID.
 * @param null $deprecated Deprecated. Not used.
 * @return string Term description, if available.
 */
function term_description( $term = 0, $deprecated = null ) {
	if ( ! $term && ( is_tax() || is_tag() || is_category() ) ) {
		$term = get_queried_object();
		if ( $term ) {
			$term = $term->term_id;
		}
	}

	$description = get_term_field( 'description', $term );

	return is_wp_error( $description ) ? '' : $description;