The Known world has cultures and a level of technology development that resembles the Europe of our Earth around 15th century. The known world does not have gunpowder or the printing press, but in many other ways the two worlds are similar.
Late-medieval feudalism is the most common system of government: A nobleman, usually a powerful warrior, rules a region of land, occupying a stronghold (often a castle) at its capital, guarding the region and enforcing his laws with soldiers normally drawn from the local population. The peasants of his region pay him taxes and services; he, in turn, protects them against invaders and evildoers.
Just as the peasants serve the noble, so too does the noble serve a greater ruler, a king or emperor. The petty noble supplies taxes to the greater ruler, and in times of war supports him with soldiers and additional money, and often must campaign with his king. The greater ruler, in turn, is obliged to protect the lesser ruler's claim to nobility, and to help protect the lesser noble in times of invasion. The oath of mutual services and support sworn between them is known as the oath of fealty.