Let's assume that each couple of rabbits give birth to two baby rabbits.

Let's count the number of couple of rabbits. At the beginning, we've 1 couple of young rabbit

F_1 = 1

Next year, they'll be adult rabbits, but won't have children yet :

F_2 = 1

On the third year, they'll a two baby rabbits, that is one couple

F_3 = 2

On the forth year, the first parent will have two baby rabbits again, but young rabbits won't be old enough to do so

F_3 = 2 + 1 = 3

On the fifth year, we've got two couples that can have little rabbits and one that cannot :

F_3 = 3 + 2 = 5

and so on... Each year, the number of couple is :

• the number of rabbits that lived the year before, that is F_{n-1}
• plus the number of rabbits that were born that year, that is the number of couple old enough to proceate, that is F_{n-2}

To conclude :

F_n = F_{n-1} - F{n-2}

Funny, isn't it ? Ok, that is not very realistic and very accurante, but who cares ? ;)