In the age of feudalism, land was a sign of wealth, and those who had lots of land were very powerful. It was not uncommon for a large landowner, known as a Lord, to hire someone known as a vassal for knightly services for a chunk of their land.
A vassal was in charge of many duties, including knighting the lords oldest son when he came of age, finding a husband for his daughter, and owed the lord a number of days of military service per year. They were also responsible for paying ransom if the lord is kidnapped.
The lord also owed a few things to their vassal as well, such as economic support for their equipment, a vow that the lord would take his vassal's side in a court of law, and that the lord would pay his vassal for his service in land.