Ultimately, entropy is a measure of the availability of free energy – of energy that isn’t hopelessly mixed in thermal equilibrium. Pump energy into a small system and complexity can thrive..
Entropy and the second law of thermodynamics has been credited with defining the arrow of time.
Have you ever been doing thermodynamics in a closed system and noticed that there’s a finite number of ways that things can be arranged, and they tend towards disorder? Of course you have, we all have. That’s entropy. And here in our Universe, entropy is on the rise.