Source

File: wp-includes/widgets.php

function wp_widget_description( $id ) {
	if ( ! is_scalar( $id ) ) {
		return;
	}

	global $wp_registered_widgets;

	if ( isset( $wp_registered_widgets[ $id ]['description'] ) ) {
		return esc_html( $wp_registered_widgets[ $id ]['description'] );
	}
}