He would do anything the noble needed to be done, and fight in his wars.
First, a noble would hand a bit of land called a fief to one of his most trusted being called a vassals in exchange for protection like a knight. The vassal must swear and oath that he will be loyal to his noble. A noble who gives a fief to a knight is called a lord. This vassal may hand a bit of land to another vassal, who we'll call vassal B and he shall swear an oath and so on. So one can be a lord and vassal.
A vassal
A vassal
A knight would often be a vassal of a more powerful noble, or subsidiary to the noble in another way.A powerful noble could also hold a knighthood himself.
vassal
The oath of a Vassal is known as an homage.The Vassal has sworn to serve another regent of a domain providing them with protection and justice.
Vassal
The oath of a Vassal is known as an homage.The Vassal has sworn to serve another regent of a domain providing them with protection and justice.
A noble was the lord ( landlord) of his lands and people but he was a vassal to the king who gave him those lands. The lands could be taken away at any time along with any titles and even his life if the king so choose to do so.
The relationship between the lord and vassal, is that the lord gives a portion of his land to a vassal which is a knight; to provide military support and protection, before doing his duty the vassal swears an oath to be loyal to a particular lord and in turn the lord grant the vassal a portion of his land and riches. If a vassal manages to serve two lords who happened to wage a battle then the vassal must continue to serve both by sending half of his men to battle for the rival lord and the other half of his men to the other opposing lord. Disloyalty results in death.
Do something VERY valuable to the sitting government
It was called the feudal system, a knight would promise the lord loyalty and protection in return for land which would make the knight a vassal. The vassal and his family now fought for the lord and they were always loyal.