system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is heat and/or energy?

It never really occurred to me before, but heat doesn't really make sense to me. As in; what is it? We can't observe it the same way as we can observe material, can we?

I remember that particles vibrate which causes heat, and that absolute zero is when "nothing" moves, or something like that. Does that mean heat is the vibration of something? But it doesn't make sense that air can be hot too...?

Also, I read/heard somewhere (I think my teacher) that heat does not generate, it only moves. If so, then how come a room becomes warm with a fire in it? Shouldn't the room become cold because the fire draws all the heat from around it?
gpt
All matter, including air, is composed of very tiny particles called atoms.  Sometimes the atoms are joined together in groups called molecules.

Atoms and molecules can have two kinds of motion, ordered and disordered.  An example of ordered motion would be a baseball bat being swung; all the molecules in the baseball bat are moving together as a unit.  Temperature is a measure of disordered motion.  Even when the matter appears to be at rest, the molecules inside it are jiggling and bouncing around (you could say they are vibrating, but they are not vibrating in sync with one another, each one vibrates at random).  The faster the molecules move when they jiggle, the higher the temperature (this isn't quite the whole story because temperature is a measure of average kinetic energy and not average speed, but it will help you visualize what's going on).  Absolute zero does indeed refer to a state where the molecules would not be moving at all, but there is no way to put matter into a true absolute zero state in reality.

If you place matter with a low temperature next to matter with a higher temperature, the temperature of the lower matter will rise and the matter with the higher temperature will drop, until they both reach the same temperature.  We call this process "heat transfer", and "heat" refers to the energy that is transferred from the higher temperature matter to the lower.

Finally, energy is a number that physicists associate with physical laws.  This number can be expressed in various ways in different situations, but it always has a specific property.  The property is that the change in energy for any physical process is zero; energy is "conserved".