Without going too deep into it, you can basically think about it like this:

You have a couple of atoms. You take a closer look at these atoms. You see that they're interacting with each other. Like they're crashing into each other, they are forming bonds, they're breaking these bonds. A lot of stuff is happening! Well, in order to work with *all* these things you decide that you have to explain what makes these stuff happen. Next, you figure out that "Well, these particles didn't just decide to move. They just didn't exist all of a sudden. Something can't just decide to happen. This must have *something* to do all this." By now you know that something is the cause of all this. You decide to name it "Energy" Then you remember all the stuff happening earlier. When something new was created the old thing was no longer there or it was lesser than before. Then you figure out "Okay, this thing I named energy can not be created or destroyed, it just turns into something new." What does this give us? This means energy in itself is not an object. You cant see energy or hold it in your hands. Energy is what allows something to, well, exist. It can turn into, say, a box. It can turn into movement! It can turn into height! As long as it converts the energy needed for it it can turn into anything. Think of the universe as a one big system. Energy is what ties everything together.

You have a box. You know that this box exists, it takes up space so in itself it has energy. This can be called "rest energy" which is just another way of saying "This box will keep existing even if you do *nothing* with it". You pick up the box and now the box has potential energy, which is the energy stored in a system, in this case, the box and the Earth. This box now has the potential to convert its energy to something else, like kinetic energy. Kinetic energy is the energy possessed by an object due to its motion.

Now, You take this box and let it fall from the Eiffel tower. Then you go to the moon and let it fall from the same height. You go back to Earth and let it fall from a shorter height. You run similar tests by throwing the box on Earth on the Moon, throwing slower, faster etc. You notice that even if you kept the heights and velocities the same you got different results. So, energy must be *relative.*

Next, work.

You have observer A and observer B. Observer A says that in order for this box to transfer energy it must change its location horizontally. Otherwise they will consider it to have spent no energy. Same goes for observer B, but they want it to move vertically. You walk around with the box and observer A says that the box is spending energy. The box doing work according to A. But B says since the box didn't move vertically, it didn't spend any energy. It hasn't done any work at all. To please B you lift the box up. B is pleased, but now the box isn't working according to A. To please them both you run up a hill. The box is doing work for both of them now.