"Development" isn't really a feature as such. It's just a number. It's the sum of the base tax and manpower (which were already in the game before 1.12), and the new base production. Patch 1.12 adds the new base production value to the game, while the Common Sense DLC enables the ability to manually increase these values with monarch points. The total Development value isn't used for much besides being one of the main factors affecting how many building slots a province has.