what is efficiency and give me two simple examples.?

Answer 1

Efficiency is the ratio of useful energy out to energy in, it is given by

Useful Energy Out/Energy In

Answer 2

The above answer is one example. Here is another.

The optimist says the glass is half full.
The pessimist says the glass is half empty.
The engineer says the glass is twice as big as it needs to be.

So the efficient glass is half as big and completely full.