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.
Entropy. It’s this super-popular idea (which is good) which is often misunderstood (which is bad). It’s the reason an icecube melts in a glass of warm water. It’s the reason you can’t get an engine which is 100% efficient.
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.