Feudalism was a way that people in the olden days—like way, way back when there were kings and knights and stuff—organized their society. Under feudalism, the king owned all the land and gave parts of it to his most important knights and lords. These lords would then let people live on the land in exchange for things like crops, sheep, or coins.
Different people had different jobs and responsibilities in this system. For example, a knight might be responsible for protecting the lord's land from invaders, while a farmer might have to plow the fields and grow wheat to give to the lord.
These responsibilities were called "feudal duties." They were basically the things that the lord or knight or farmer had to do in exchange for living on the land. These duties were really important because if someone didn't do their part, they might lose their land or even their life.
So, to summarize: feudal duties were the responsibilities that people in the feudal system had to do in order to keep their land and stay safe. It was like a big agreement between people with different jobs and responsibilities, and it helped keep everything organized and running smoothly.