It means, in brief, that the subject is not eating normally.
Lance proffers a bagel with cream cheese.
The subject refuses it. Lance proffers a chunk of challah bread. At most, the subject will try a bite, announce that it tastes like sawdust, and refuse to eat any more. At times, the subject will choke down a pre-measured quantity of food, complaining all the while, moved to such heroism by the knowledge that one must eat to live. At times, the subject eats normally.