Energy is neither created nor destroyed
To scientists, conservation of energy does not mean saving energy. Instead, the law of conservation of energy says that energy is neither created nor destroyed. When people use energy, it doesn't disappear. People change energy from one form of energy into another form of energy.
A car engine burns gasoline, converting the chemical energy in gasoline into mechanical energy. Solar cells change radiant energy into electrical energy. Energy changes form, but the total amount of energy in the universe stays the same.
Converting one form of energy into another
Energy efficiency is the amount of useful energy obtained from any type of system. A perfectly energy-efficient machine would convert all the energy put into the machine to useful work. In reality, converting one form of energy into another form of energy always involves a loss of useable energy.
Most energy transformations are not very efficient. The human body is a good example. The human body is like a machine, and the fuel it requires is food. Food gives a person energy to move, breathe, and think. But the human body isn't very efficient at converting food into useful work. The human body is less than 5% efficient most of the time. The rest of the energy is lost as heat.