A lot of good answers have been given pertaining to this not being an absolute concept. Still, there was a traditional order that, by and large, remained accepted in many regions. If we stick with, for example, medieval Europe , the example of Henry II before Toulouse may be instructive.

The Angevin king Henry II ruled one of the most powerful kingdoms of 12th century Europe, and this created a fair amount of tension in the complicated diplomatic web that was feudal Europe. So in 1159, the king of England had set his sights on conquering Toulouse, a piece of territory disputed between him and count Raymond. Now Raymond was the brother-in-law to the king of France, Louis VII. Louis was worried since Henry - also his vassal on the continent, as he was only king in England - was already so darn powerful, acquiring Toulouse would only strengthen his position. Yet ultimately, he made a genius move. As Henry II arrived before Toulouse at the head of one of the largests contemporary armies, his heart sank when he saw the banner hoisted over the city: Louis VII had taken up quarters inside before his army arrived...

The game was up, and Henry knew it. He ordered his army to ransack the countryside and went back home, failing his ultimate objective. So, what happened? Why did Henry hold back & why is this instructive for your question?

Because Henry had sworn fealty to Louis (for those lands for which he was beholden to him). Henry himself had more than enough rebellious lords under him, keen to resist his royal authority. To attack Toulouse with Louis inside was to break all his oaths he had sworn (under his god), making him that thing medieval society as an honour-culture despised so hard: the honourless oathbreaker. So Henry knew that if here and now he pressed his claim on Toulouse and violated the person of Louis VII, he would enable each and every lord who held a grudge towards him from Scotland to the Pyrenees, to safely renounce their allegiance and rebel.

Medieval society, like many, was structured on concepts of honour, typically expressed through oaths and the like. People took these seriously, for breaking the social contract - then as much as now - renders one open to the fallout.

So many things underpinned royal authority, from the more raw aspects of money and power (soft or hard) to the those of concepts of legitimacy.