What does it mean when an inmate gets: unsupervised work release? Are they allowed to work during the day and then must return to the prison?

This sounds more like a halfway house arrangement. However, if he is still in the prison, then he has a very trustworthy position, one that should make you feel good about the progress and programming that he has done. There are VERY few instances of "unsupervised work release". We see this sometimes in the minimum camps of the federal prison system (BOP) where inmates are working mostly unsupervised on many of the military bases around the country and then coming back to the camp in the evening.