Complex sentence: L'Île-de-France, like the other French regions, was turned into a "territorial collectivity", i.e., it is no more a mere administrative structure, but a full-fledged political entity, on a par with the departments and communes.
Split into simpler sentences: L'Île-de-France, like the other French regions, was turned into a "territorial collectivity". In other words, it was transformed from a mere administrative region of the state to a full-fledged political entity, on a par with the departments and communes.