When a king granted land to a lord the lord in return promised to?

fight for the king and serve them

2nd Answer: To expand that a bit, when a greater noble (was not always a king) granted land to a vassal the vassal could take on a number of responsibilities, including military service, hospitality for the lord and his servants, attendance of the lord's court to give advice and council, and participation in the lord's entourage. In the later middle ages the demands for physical military service could sometimes be converted to a monetary payment. This was known as scutage.