There is no universal definition for what “free will” is. However, most people would define it as our capacity to act independently of the influences of our external environment. In theory, free will is what drives all of our decisions — like our morality or sense of self.
Free will doesn’t exist under the lens of hard determinism. Hard determinists essentially believe that everything that happens in our universe is capable of being predicted — that nothing is truly random, but instead has a concrete cause, no matter how tiny. Because everything has a direct cause, the human consciousness and decision-making are merely physical reactions to physical stimuli, internal and external. There is no “free will”, just tiny reactions.
“Escape from Spiderhead” is frequently said to be a literary proof of free will — a demonstration that consciousness couldn’t possibly be limited to the reaction of chemicals, specifically using the example of pharmaceuticals, in our brain. But it fails to account for the fact that Jeff’s decisions and feelings are still the result of reactions to stimuli. To Saunders, Jeff might feel that he loves the women but not “really” love them like one would typically define love because of his “free will”. He might have been a murderer, but he is no longer that due to his “free will”. Yet he fails to demonstrate that these realities are not each the result of a pre-existing stimulus.