Energy Efficiency
Energy efficiency is how much useful light or work you get from each watt of power used.
Think of it this way
Two pots of water sit on the stove. One pot has a tight lid; the other has no lid. You add the same heat to both pots. The lidded pot boils faster because it traps more heat inside, while the lidless pot loses much of its heat as steam.
Explaining energy efficiency by grade level
Two lamps sit side by side. One uses an old bulb. The other uses an LED bulb. The LED makes the same bright light but needs less power. It wastes less energy as heat. That means the LED does more with less.