http://ckiiwiki.com/De_jure
De jure territory is important to be able to expand. For example; if you own the duchy of Middlesex but not the county of london, you will get De Jure claims on london, and thus be able to conquer it. De Jure borders is a map mode you can turn on/off, you want to keep your vassals within their prober de jure borders otherwise they will just fight all the time.
My advice to you, start up the game as a duke in Ireland, or the King of Castille if you don't mind starting in a risky posistion. Then check out each screen in your goverment interface, hover your mouse over any icons there may be and read the text, there is a lot of good info to be found there.
