We don't know what energy is. That's the point. Yet it is being expressed as a whole array of differentiated quantum phenomena. Or at least, so we think at the moment. We call the array of quantum phenomena "quantum particles" but they aren't really particles at all. Not in any material sense.
For day to day reality, energy is ability for any object (matters) to do work.
And that (energy) require mass.
Some mass are gain and some mass are loss - hence a trade off...which in turn cause gain in energy and loss in energy, which is another trade-off. There are always trade offs, whenever there are work to be done.
Mass and energy are related, and they are both properties of matter or an object.
Take for instance, a marathon runner, require certain mass, in order to have the energy to complete the run.
If you look at a person’s body masses, about 20% are proteins (eg proteins are found in cells, tissues, muscles, etc), 12% are lipids (body fat and fatty acids). But most of human’s mas come from water, about 65%.
When runner use their muscles to run, hence doing work, the store energy (potential energy) in the mass, will become kinetic energy. But some of the potential/kinetic energy are loss in the form of thermal energy, meaning heat. The water in our body is what keep us not only hydrated, but keep body cool. But since we expend energy doing work, some of those masses I talked about, are loss though sweating. So unless we replenish the water we loss through sweating, thereby causing the body to suffer from dehydration, it can cause heat exhaustion, and marathon runner could collapse.
And losing masses, like water in our bodies, mean losing energy.
But just as there are loss in mass and energy, while human body do work, we also gain some mass and therefore gain energy as well, when we breathe in air. Our breathing system, convert mass of air into energy, and can keep runner going.
Like I said losing mass will mean losing energy, but you can regain energy by consuming masses, like when we eat, drink and breathe. Our body needs to replenish any mass loss.
So there is trade off, gain vs loss in any physical systems.
There are two things in the universe that I know of, that don’t have mass: in particle physics, these are called gluons and photons. They both don’t have electric charge.
You might know photons as light or radiation.
Photon, while mass-less, does have energy that cause photons to move or propagate through space, moving at the speed of light. Their energy come from electromagnetic fields, which exert electromagnetic force on photons, hence the ability of light to travel through space. The energy comes from electromagnetic fields.
It is more complex than that, so I am simply laying my examples out about where energy come from.
If you want more expert explanations then ask Valjean, exchemist, polymath257, who have better grasp for physics.