Why Spoon? There is a great article on what it is like to live with chronic illness. It originates from Christine Miserandino's Spoon Theory story, in which she explains what it's like to live with chronic illness to a friend, and uses a handful of spoons to demonstrate. For example, daily activities that healthy people tend not to think about take tolls on people with chronic illness. You start the day with a certain number of spoons, and play your day around not running out. Socializing might cost spoons, using the stairs might cost spoons, lifting things might cost spoons.
The purpose of this app is to mimick tracking "Spoons" in order to help track an minimize symptoms for people suffering with chronic illness.