The second law of thermodynamics states that energy is conserved in every transformation, but never perfectly preserved in its usable form. Every time energy is transformed, some portion of it degrades into a less useful form, typically low-grade heat that disperses into the surroundings. This is why processes tend not to run themselves backwards. The energy that is needed to reverse the process is no longer available in a useful form. Most often it is thrown off into the environment as heat. This spreading out of usable energy into unusable, dispersed energy is called entropy.
In other words, when energy is used to create or maintain order in a system, a greater degree of disorder is produced elsewhere as a byproduct. For example, a refrigerator creates cold, ordered air insight, but dumps even more heat than it removed from the fridge into the room it is in (disorder).
Local order is always paid for with a larger increase in disorder somewhere else. The insight is that all systems (machines, organisms, companies) require continuous input of energy to resist the inevitable drift toward degradation and disorder. It is why we need to eat to stay alive, as we use chemical energy in food to maintain order in our complex, highly organized bodies.
Connections
Link Explanation: The linked note describes how words are a layer of abstraction over reality that we use to communicate, but will always fail to fully capture that reality. This is a useful metaphor for entropy (though it is closer to information theory than thermodynamics).
Imagine a real life game of “telephone” where one person thinks of a scene, then must describe that scene in words to another person, who again describes that scene to the next person, and so on. Every information transaction introduces loss and therefore drifts further from the source. Inventions that reduce that loss (or entropy) and allow information to flow faster, or more clearly, are what we value. To extend the example, if the first person were to take a photograph of the scene and simply pass that along the chain, rather than describe it in words, the final person would have a much clearer idea of the original scene than in the original example.