To my understanding "burn out" is a term borrowed from mechanics to describe a similar situation in the physical, mental state. Over use of a motor will cause it to over heat to the point that the insulation is damaged or the brushes break down. Gone, kaput, buy another.
Burn out in humans is similar with the difference that with rest you usually get a second chance, unlike your cordless drill for instance.
We all have different degrees of reserves so some people burn out easier than others. With Arthritis burn out manifests itself as a flare, a lack of reserve to use to go on. Rest and it comes back.
Same with the mental. Anxiety and worry can stress all parts of the nervous system to the point that they don't work, or work erratically. A word that comes from the root error. So a person can use these erratic thoughts to either signal rest or panic. Although rest is harder to do than panic it is the more appropriate thing to do.
The body will take a lot of abuse and spring back. Continuous abuse can lead to failure forcing rest.
Hence that old term "nervous breakdown". Burn out is what it really is.
Burn out is when you just can't take it any more.
Whether physical or mental, give it a rest. By the way using different muscles or mental pathways can prevent or at least forestall burn out.
Burn out can get so bad that you need medical intervention to force yourself to rest. One should not let it get that far.