This is an excellent question and it's one which very rarely gets asked. Energy is a notoriously difficult thing to teach because a lot of people (even Scientists) are taught it incorrectly in the first place, so they pass on a faulty explanation to their students.
Let me start by saying what energy is not. It's not a substance. When an object "gains energy" this does not mean it's somehow got more stuff to it. An object higher up has gravitational potential energy, this doesn't mean you could somehow look at it and count the "energons".
Energy, defined as simply as possible, is "the ability to do work". Work means Force x distance moved in direction of Force. Force is a mass changing its velocity and velocity is a description of what something is doing. So, going backwards, Force is a measure of making something change what it's doing. Work is a measure of how much we've changed the thing, and Energy is a measure of the ability to do that.
If this sounds a bit abstract then it should. Energy is an abstract concept and we don't need it for any explanation. Ever. Unfortunately people use the term energy so often that they stop thinking about what it actually means and say things like "this happened because of Energy" or "Energy is what made it happen". This is cheating. Besides being a misuse of the word Energy.
For example, we might refer to fuel being burned, releasing its energy, making the car go. But this is not an explanation. This is like saying "the thing which makes the car go, is what makes it go". The thing making the car go is the movement of the particles and their collisions with the interior of the engine block.
The fuel particles are vibrating around a lot as they smash into each other. As the particles bump into the inside of the engine, the electrons in one atom repel the electrons in the other atom causing an overall repulsive effect, pushing the engine (and anything attached to it) forward, and the reacted fuel particles backward.
The question is how much fuel will lead to how much movement? This is what we use Energy for. We can measure the fuel's potential to react as its chemical energy. We can measure its speed using kinetic energy. We can measure the interactions between the colliding atoms as electromagnetic energy and so on. So, if we have (for each part of the process) an equation which translates the actual thing happening into a common unit (the Joule) we can easily move back and forth from process to process. The actual events happening are the particles moving and interacting, not "Energy being released" or "Energy changing form".
Energy should not, strictly speaking, be used as a noun. It should be used as an adjective because it is a measurement/description of something. Any time we talk about Energy in a Scientific concept, we have to remember that we need to qualify what the energy relates to. To say "this particle has energy" is meaningless. To say "this particle has kinetic energy" or "this particle contains a certain number of potential photons" is meaningful.
What Energy actually is, is a useful way of keeping track of cause and effect. It's a physicists way of quantifying "how much of this cause" will lead to "how much of this effect". In Einstein's infamous E=mc^2 formula, the E is not referring to some actual thing called energy. It is referring to the kinetic energy of massless particles (photons).
Another way to think of it would be to look at an object, a pencil say, and describe it as having a length. Length is not a substance, but a measurement telling you how much of the Universe the pencil takes up. The actual thing going on is particles are bonded together to form the shape of a pencil. We can quantify how much pencil there is by talking about its length but we cannot treat "length" as a substance and release it from the pencil or somehow give it more. It can't be created or destroyed for instance. You chop the pencil in half and you still have the same amount of length, just distributed differently. It's the same with Energy.
Ability to make things happen is what Energy is and you can't create it out of nowhere (a Physicist's way of saying: things don't happen without a cause) and you can't destroy it (you can't have a cause without an effect). So always remember that although Energy is a useful tool for keeping track of things, it is not a substance and never an explanation. Even when you hear distinguished Scientists talking about it as if it's an actual thing, remember this is just shorthand vocabulary.
Energy = ability to make a change to the Universe.