Spoon theory explains why some days we have more capacity than others. It was Christine Miserandino who came up with the idea in 2003 to explain chronic fatigue syndrome. Neurodivergent people find it useful too.
Consider your capacity today:
Physical capacity - how is your body feeling? Pain? Fatigue?
Brain capacity - do you have brain fog? Are you thinking clearly?
Social-emotional capacity - how much people time can I handle today?
The idea is to not ended up more exhausted tomorrow - unless what you are doing today is important enough to do it and then have a recovery day tomorrow.
Spoon theory helps you understand yourself and to communicate with others.