Assuming you're referring to the French expression "bonne ville," French Wikipedia explains that:
Classiquement, une bonne ville est dans l'Europe du Moyen Âge une ville bénéficiant d’un statut particulier marqué par une relation privilégiée avec le roi, ce qui implique un degré minimum d’indépendance par rapport à d’autres seigneurs.
Or, in English, "Classically, a 'good city' in medieval Europe was a city enjoying a special status marked by a privileged relationship with the king, one which implied at least a degree of independence with respect to other lords." It goes on to say that under the First Empire, the term was revived as an honorific designation. In 1821 this designation was granted to 40 cities: Abbeville, Aix-en-Provence, Amiens, Angers, Antibes, Avignon, Besançon, Bordeaux, Bourges, Caen, Cambrai, Carcassonne, Colmar, Cette, Clermont-Ferrand, Dijon, Grenoble, Lille, Lyon, Marseille, Metz, Montauban, Montpellier, Nancy, Nantes, Nîmes, Orléans, Paris, Pau, Reims, Rennes, La Rochelle, Rouen, Strasbourg, Toulon, Toulouse, Tours, Troyes, Versailles, and Vesoul.