Long before I knew I had ME, or even what ME was, I used to have a fairly physical job. On one occasion I had to empty a full 40 container of office furniture, panels etc. and carry it upto the 4th floor (no access to lifts as it was a new block). I did this easily in a day, with nothing more than the odd sore muscle the next day, because I was allowed to go at my own pace, there's the job, get on with it. On other jobs I haven't had that luxury and have been completely wiped and unable to continue after 2 trips to a lower floor, and then been off sick for a week. I have, or had, a limit on the amount of energy I can use at any one time, exceeding that limit causes "problems", if I didn't exceed it I could carry on like that indefinitely. I may not use the same words as
@alex3619 used but I was aware that if I used energy faster than my body could produce it then there were consequences, and other people didn't seem to have this limitation, but also didn't have my endurance, provided I stayed below this limit.
To give another example I've walked across Cornwall several times in a week, it's not a big county but I must have covered 2-300 miles, with no problems, apart from there being cows all over the place. But at the same time walking a few hundred meters double time, could leave me needing time off work.
This is what I assume is meant by "metabolically derived energy".