No device transfers all of its input energy usefully. Some is wasted, usually as thermal energy that spreads to the surroundings. Efficiency compares the useful energy you get out with the total energy you put in.
The equation is: efficiency = useful energy transferred by the device ÷ total energy supplied to the device. Both energies are in joules, so the units cancel and efficiency has no unit. The answer is a decimal, or you can multiply by 100 to give a percentage.
Worked example: a motor is supplied with 200 J and transfers 150 J usefully. Efficiency = 150 ÷ 200 = 0.75, which is 75%. The wasted energy is 200 - 150 = 50 J. Efficiency can also be found from power, using useful power output ÷ total power input. Because some energy is always wasted, efficiency is never more than 1, or 100%.