King of the Franks, Charlemagne, was one of many kings who used the feudal system. As king he owned all the land of his Frankish kingdom.
When he gave parts of this lands to his lords they were called the king's vassals. They not only received the land, but also the serf farmers who lived on that land. The vassal could live off the produce of the awarded land and keep himself busy with military training, warfare and jurisdiction.
The group of vassals together formed the rich nobility or aristocracy.
With titles as count, duke or baron, they ruled parts of the land owned by the king. Sometimes vassals gave parts of "their" land to lower vassals: knights, in return for protection and military service.
Study the overview in the next slide.