With a gasoline generator, you expend 100 units of (chemical) energy to obtain (maybe) 30 units of electrical energy. Efficiency=30%.
With a wind generator, you expend zero energy(actually you use wind energy, but you don't pay for that) to get your 100 units of electrical energy. Efficiency = 100/0 (infinity)

Efficiency is one of those words that has lost most of its meaning, but is widely employed.