What Does Energy Efficiency Mean?

Energy efficiency is the amount of energy converted to a useful purpose as a percentage of the energy that was used by the device in the first place.  For example, a 100 watt light incandescent light bulb uses 100 watts (not surprising) but about 90 watts goes to produce heat and only about 10 watts (or less) goes to produce light.  So then the bulb is 10 % efficient.

It refers to the amount of energy being used by an appliance for its specific purpose. For example with light bulbs, a higher energy efficient one will transfer a higher percentage of the energy put into it into light energy as opposed to heat energy. Whereas a lower efficiency one will transfer a higher proportion of energy into heat compared to the high efficiency one.
 
So a higher energy efficient bulb will use less energy to put out the same amount of light as a lower energy efficient one.

Energy efficiency is the percentage of total energy input that is consumed as useful energy and not wasted energy

Energy efficiency  means to use energy more wisely/smartly.