It is a system of ruling, in which ruling starts off by a member of a family, and continues till the end of a family. Monarchs were very common in history. A lot of countries used to have a monarch, but no longer have them. Some countries that still have monarchs have other leaders that actually have the power.
Monarchs tended to have monarchs of lower honour below them, e.g. a Empire could be split into several kingdoms, and that kingdom would be split into several duchies or principalities, nations would rarely include all titles, usually going from duchy to kingdom instead of duchy to principality to kingdom. Prince/Princess means 'Ruler of a Principality' and 'son of king/queen', in some other languages there are separate words for them.
|Grand Principality||Grand Prince||Grand Princess|
|Archduchy||Arch Duke||Arch Duchess|
|Grand Duchy||Grand Duke||Grand Duchess|