Entropy
Entropy is a measure of how spread out and mixed up energy becomes over time.
Think of it this way
A jar of red beads sits on one side of a tray, and white beads sit on the other. Tip both jars and let the beads spill. Red and white mix across the whole surface in seconds. That mixed-up state is entropy. Things move from neat to scattered on their own.
Explaining entropy by grade level
Stretch a rubber band and hold it to your lip. It feels warm. Let it snap back and it feels cool. Energy spreads out each time you stretch or release it, and things never go back to exactly the same.