'Post-game content' is the term commonly used to refer to gameplay that takes place after the main storyline has been completed.
Such content for Dragon Quest IX includes Quests and missions, which can be obtained either via DQVC or by completing other quests. Most of them build up, and can be only started once you've completed the previous one in the chain, creating a sort of separate storyline. At the end of each 'storyline' there will normally be a boss fight. Some shops, locations and items are only found post-game.