Taken from Wikipedia:
The spoon theory is an analogy originally used by some people with a disability and people with chronic illness to describe their everyday living experience when their disability or illness (physical or mental) presents in a reduced amount of energy available for productive tasks.
Spoons are an intangible unit of measurement used to track how much energy a person has throughout a given day. Each activity “costs” a certain number of spoons, which will only slowly be replaced as the person “recharges” through rest or other activities that do not require (or even refill) spoons. A person who runs out of spoons loses the ability to do anything other than rest.
One of the tenets of the spoon theory is that people with disabilities, chronic illness, or mental illness must plan their daily activities to conserve their “spoons,” while healthy, able-bodied, people have a “never-ending supply of spoons” and thus have never needed to worry about running out. Because healthy people do not feel the impact of spending spoons for mundane tasks such as bathing and getting dressed, they may not realize the amount of energy that chronically ill or disabled people may have to get through the day.