Definition
Feudalism was a relationship in which a king or powerful lord entrusted part of his land to someone below him, and in return received loyalty and military help. The person who received the land ruled it and lived off it, and when called, had to show up with a horse and weapons. It was a way of running society that spread widely in Europe long ago.
No One Can Look After a Huge Area Alone
Imagine your class has been put in charge of the whole schoolyard at the school festival. The class president can't watch over such a big space alone. In the end, the yard gets split into zones, and each friend is given one.
The friend in charge of a zone gets to keep the money earned there. In return, they promise to rush over and help if something goes wrong on festival day. It is a trade of a post for help.
A king in Europe long ago was in a similar spot. In an age of poor roads and poor communication, there was no way to rule every corner of a wide land himself. So he broke off pieces of land and entrusted them to powerful people.
The person who received land kept the grain and taxes it produced. In return, when war broke out, he had to lead soldiers and rush to the king's side. This trade of land for loyalty is the backbone of feudalism.
The Promise Runs Downward
The friend in charge of a zone might find it too much to handle alone, too. Then they split up posts inside the zone and hand them to other friends. The first promise gets passed down, one step at a time.
Feudalism built up layers the same way. A lord who received land from the king handed part of it on to knights. A knight swore loyalty to his superior, his lord, and went off to the battlefield.
Below the knights was one more layer of a different kind. Peasants who farmed the lord's land handed over part of their harvest and their labor, and in return could ask to be protected from bandits and the dangers of war. But this was not the lord-and-vassal bond of receiving land and giving loyalty and military help. It was a separate relationship called the manorial system.
Here is something worth noticing. This relationship was not a setup where orders only came down from above. It was a promise in which each side had a share to keep. If those above broke the promise, those below sometimes turned their backs.
To Be More Precise
Just as every class might split up the festival zones differently, feudalism looked different from place to place and from era to era. In some places the king was strong. In others, the lords were just as strong as the king. It is closer to a name for a bundle of similar arrangements than a single, fixed system.
There is also an overlap that came from translation. An old system in East Asia, where land was handed out to be ruled, and this European arrangement were translated with the same word. It is a case of systems with different roots ending up with one name.
It is also hard to say that rank was set in stone. Some people became knights by earning distinction in battle, and some peasants moved to towns and became merchants. But such moves were not common.
Still, the big picture comes back to that first promise. When a land is too wide to rule alone, society ran by handing out land and receiving loyalty in return.
Common misconceptions
In feudalism, the king was a supreme ruler who could control every corner of the country as he pleased.
Once the land was handed out, it was hard for the king to rule it directly. In some places, a lord's power was no less than the king's.
Feudalism was a system where those above only gave orders and those below simply obeyed.
It was a two-way promise: land and protection were given, and loyalty and military help were received. If one side broke the promise, the relationship could fall apart.
Where you meet it
A knight who guards a castle is called up by his lord and rides to the battlefield with his horse and armor.
A peasant who farms the lord's land hands over part of the harvest, and in return gets a place to take shelter inside the castle when danger comes. This is the manorial side of life, which ran alongside feudalism.
In one sentence
Because one person could not rule a wide land alone, land was handed out, and loyalty and help were received in return. That was the promise.