Maxwell's Demon is a thought experiment that attempts to defeat The Second Law of Thermodynamics, which states that the amount of Free Energy in a closed system will always decrease (ie. Entropy will always increase).
Imagine two boxes separated by a small gate. In one box the particles are hot and in the other cold. The Second Law tells us that over time, if the gate is open, the temperatures will even out. The hot box will get colder and the cold box will get hotter until they are roughly the same.
Now imagine a little demon sitting on the gate. It watches the particles, opening and closing the gate to let only hot particles into the hot box and cold particles into the cold box. In this way, over time, one of the boxes gets hotter while the other gets colder.
It seems like this system violates The Second Law.
The trick is: computation isn't free. Monitoring the state of the system and determining if the gate should be open or closed comes with a cost in Free Energy. This cost will always end up being greater than the amount of generated Free Energy.
So where did the demon get the energy? Eating, presumably. It eats food to store Free Energy within itself that it can release as computation enabling the seeming reversal of Entropy.
Living things are similar. We consume Free Energy in the form of food and drink, and then use that Free Energy to compute actions which allow us to consume more Free Energy from the environment. This process allows us to maintain our disequilibrium with the environment.