Entropy (thermodynamics)

Entropy is a way to measure how much energy can be used for work in a system. It's like a measure of the unordered energy. When entropy increases, it means the energy cannot be used to do useful work in the system. To use a simple example: if you take one ice cube and move it to a larger tray of ice cubes, the total energy of the system stays the same but the entropy increases because it's harder to move the single ice cube back to the original place because there are more ice cubes around it. The energy is still there, but it's unordered and difficult to use.