Answer:
I've read that Nobles are the second in rank (under the king) and they own a large area that they got from the king. For a return, the nobles have to promise him to serve him as if they are his servants. They need to come with an army when the king needs it, they have to give a lot of the taxes to the king. Beause, the king is still higher in rank and can take the Nobles' rank back at every time.
This is all part of the feudal system, you might want to search for that as well, it is kind of this:
Kings
Nobles
Knights
Free men
Peasants (half free people) and Slaves.
I hope this answer helped you!