Those were farmers who were not working at the time (due to periodic flooding of the Nile or whatever). I believe they paid their taxes in labor instead of grain, which earned them more money on their harvest. They benefitted from free food and the pharaoh had fewer people milling around with nothing to do (potential revolutionaries).

They weren’t slaves, they were just normal farmers paying their taxes. They could probably just leave if they had to. I seem to remember we have records of their attendance. Some people were not present and the reason is like “drunk” or something.