How were the knight and serf similar?
The serf and knight appear very different on the surface. Serfs were poor and tied to the land, eking out a meager subsistence by agricultural labor on the land to which they were attached. They had few rights or freedoms, were unlikely to be educated, and had no real opportunities for advancement. Knights were members of the nobility, well-educated, generally well-off, and served the king primarily as heavily armed and skilled warriors. They owned substantial estates.
The main similarity between the knight and the serf is that they were both parts of medieval society. In theory, within the feudal system, all subjects were vassals of the king, and only held land at the king's pleasure in return for service to the king. Both serf and knight, therefore, in theory were equally under the absolute power of the monarch.