I would say that they provided military service for their lord.
Feudalism refers to a system where a person had land, and people who worked on his land in return for something. Those people had to work, as well as to provide protection to their lord, and in return they could live there and support their families. Similarly, although knights didn't work on their lord's land, they did provide protection to him - by being in his service, knights had to protect their lord in case of danger.
