A more extensive explanation:
A farmer buys two newborn rabbits at the marketplace in Pisa: a male and a female. The first month this couple does not produce little ones, but after the second month the first two rabbits are born: a male and a female:
At the end of each month of the following months this first couple brings into the world two rabbits, one of each genus. Newborn rabbits start to mate after two months and give birth to a couple of little ones each month.
The Fibonacci sequence 1, 1, 2, 3, 5, 8, 13, ... shows how many rabbits the farmer has in the successive months. We suppose that the rabbits never die.