Entropy is one of what I consider a mystery of the universe (along with Dark Matter and Dark Energy). Entropy itself is not a mystery, but rather why does it only ever increase? And why was the Big Bang a moment of tremendously low entropy? How did entropy reverse itself?
Entropy reversing is the central theme to Isaac Asimov’s short story “The Last Question” (You can read it here: https://www.physics.princeton.edu/ph115/LQ.pdf)

In the short story, humanity rises, falls and transcend all without being able to figure out how did entropy reverse itself, even with the help of the super computer Multivac. Multivac started off as a super computer, but as the story continues, it becomes a transcendent universe computational entity, that once humanity has faded, is able to figure out, Why did entropy reverse itself at the beginning of time.

Entropy is defined as: “Entropy is a thermodynamic quantity that measures the amount of disorder or randomness in a system. It is a measure of the energy dispersal in a system. Low entropy means that the system is ordered, while high entropy means the system is disordered.”

In short, you can think of it as a measure of how “spread out” energy is in the universe. At the beginning of time, it was concentrated, at the end of time, energy will be evenly spread out across the universe.

Check out my video on Isaac Asimov’s “The Last Question” and Entropy: https://youtu.be/axcrALXx5gg