IRL? Legally. It's usually paired with de facto, which means what actually happens. So de jure the King of France is overlord to Normandy, but de facto the King if England is in charge. If the French and English go to war de jure the Normans are supposed to fight for France, but they won't actually do that (they'll fight for their Duke, who also happens to be King of England).
In the context of CK2 de jure usually refers to the map-files. Each province is part of a Kingdom, and a Duchy in the map-files. So if the map-files say a County is in the Duchy of Essex, and the Kingdom of England it is de jure part of the Duchy of Essex and Kingdom of England. The game is designed so that each Duke and King can get control over his de jure provinces a lot more easily then non-de jure provinces.
If a Lord who is not de jure Duke of Essex owns half the provinces in Essex he can usurp the title, making him rightful Duke of Essex (note: the old Duke gets a claim). If there is no Duke of Essex he can create the title, at which point he'll have multiple opportunities to finagle the other half of Essex's provinces, including claims on them, events, etc.
Nick