There's a woman named Christine Miserandino. She has Lupus. She came up with The Spoon Theory to explain to people what's it's like to live with a disability or chronic illness. Her website is www.butyoudontlooksick.com.
The Spoon Theory explains what it's like to have to prioritize and choose which activities you do during the day and which ones you exclude because sometimes the simple things take up more than their fair share of energy when you have little energy to spare and more pain and immobility than others.
While I have to watch what I do, on a scale of 1-10, I'm a 2. I'm lucky. I don't have (yet) a lot of the issues that many of my friends with MS have. The Spoon Theory applies to everyone who has some sort of physical difficulty. Some of us just start off the day with more spoons than others.