Efficiency is the 3BV of a board divided by the number of clicks used to solve it. The less clicks used, the higher the efficiency. An Efficiency of 100% means you solved a 3BV 50 board in 50 clicks.
The only way to gain efficiency is to chord. Efficiency is lost by placing flags, wasting clicks, or clicking cells that would be revealed by an opening. By combining NF technique and smart chording you can get more than 100% efficiency.
Here are some examples:
A good way to learn is to watch high level efficiency arena gameplay: