A common confusion is that the term "energy" can mean two things: something useful for doing work, and some "level" for lack of a better word.
Imagine two situations.
First, you are on Earth, and have a lot of hot air. 500*C hot.
You have a source of energy - as that air cools, it can drive mechanical machines, or you can use peltier/termocouple devices to turn the heat to electricity, and so on.
The hot air have energy, of the useful kind.
Second, imagine that you are on Venus.
You have the same supply of 500*C air all around you.
However, there is no useful energy to be had, since all the air is equally hot around you.
This is the energy of the "energy level" kind.
There have to be a difference in energy levels for it to be useful. I.e. you can stick a radiator out into space from Venus, and that hot air would suddenly becomes useful.
There is an energy level difference between a wound spring and an unwound one, between alcohol molecule and the water and CO2 ones it burns into, between 4 hydrogen atoms and one helium atom they can fuse into, and so on.
"Energy source" means something that would exhibit that difference.
Oil and coal that can be burned into simpler molecules.
Hydrogen that can be fused, or uranium that could be fissioned.
Heat from the Earth interior, contrasted with the cold of it's surface.
That's what "energy" mean.