And under normal play, you will usually have to hand off duchies and other titles to vassals to keep them happy. The kingdoms all have an upper limit on their size to prevent a single person from gaining too much power. You can only hold so many landed titles before you start to incur penalties all over the place. they will become more and more important as the game goes on.
Vassal Limits are important, if you go over this, you will incur tax and opinion penalties. These are the ones that affect Vassal Limit. In usual terms, they hold a title beneath yours and are subservient to you. These will often demand landed titles and council seats. Expect them to challenge you politically, or even declare war for independence. Knowing each of these and what to expect will help you in the long run Vassals come in a few different types, depending on their power and position.
