Spoon theory is a conceptional model of how stress effects different people. A 'spoon' is a subjective unit of mental energy, and losing a 'spoon' saps your energy, making it harder to deal with the various tasks that one must perform throughout the day.
This model was proposed by Christine Miserandin on her blog But You Don't Look Sick. She describes eating dinner out with her friend and being asked what it was like to live with lupus. Looking around for an apt metaphor she decided, arbitrarily, to go for the spoons. 'Spoon', in this case, is simply a convenient label for something we don't usually bother to label -- mental energy, ability to deal with stress, ability to engage in daily activities without stress. Healthy people expect to have an infinite number of spoons; they can absolutely make it through an average day without having to worry about hitting the wall. Living with lupus, on the other hand, one loses energy doing simple things such as making breakfast, taking a shower, getting dressed... the list goes on.
And as each activity takes extra time, effort, and planning, each activity costs you spoons. And unless you are careful, you run out. And then you're done.
There are some minor problems with this theory, and I'm going to get them out of the way now so no one gets hung up on them: clearly, this is not a 'theory' in any strict sense of the word. Clearly, it has nothing to do with spoons. And clearly, almost everyone has spoon problems of some sort, not just those with lupus.
None of that is important. You can absolutely call it the 'foobar model of humanity' if that makes you feel better. What is important is that this model fits really well with what millions of people are feeling -- those with lupus, fibromyalgia, chronic pain, social anxiety disorder, autism spectrum disorder, sensory integration disorders... and hundreds of other conditions.
As the term 'spoon theory' is taken up by more and more groups, it has moved away a bit from the original usage in But You Don't Look Sick. Depending on who is talking, a spoon might be burnt doing almost anything, and might be recharged by almost anything. Christine Miserandin described taking a shower and cooking as exhausting, and sleep being her way to recharge. She did not list sunlight, unscheduled free time, or talking to her friends as things that lose spoons, but for some people they are. Likewise, for some people cooking a meal or taking a shower may help them regain a spoon.
This flexibility is perhaps the most important part of spoon theory. From day to day, the number spoons you have will vary, and the spoon expenses and credits will vary. Things that would normally save you a spoon unexpectedly cost you a spoon (sleep recharges; insomnia drains. Cooking your favorite meal recharges; finding that you are out of onions ruins everything). And because you are non-typical, you are in charge of learning and managing your spoons -- the solutions that work for most people do not work for you, or you wouldn't be worrying about spoon theory. Trying to describe exactly what a spoon is can be nearly impossible... which doesn't make them any less real, or any less relatable.
It is starting to become common for people who suffer from chronic illnesses, including mental illness, to refer to themselves as spoonies in reference to this theory.