The correct answer is - loyalty.
The best synonym for the word ''fealty'' in this sense is ''loyalty''. The vassals and the lords were dependent on each other. The lords had greater authority and individual power, but if the vassals united than they had larger power. On the other hand, without the lord to control, mediate, and unite them, the vassals would have most likely fought between each other thus losing that little individual power that they have.
In order for the system and the hierarchy to function properly in the best interest of both sides, the vassals were swearing oaths of fealty, or rather loyalty, to the lord, thus being in his service. The lord too needed to give some benefits to the vassals for their loyalty and support, usually wealth or land.