Very simply, Fibonacci are mathematical ratios that price pulls back to before resuming the trend. The four major Fibonacci retracement levels are: 0.236, 0.500, 0.618, and 0.786.

Fibonacci sequence explained. The Fibonacci sequence is a series of numbers in which each number is the sum of the two that precede it. Starting at 0 and 1, the first 10 numbers of the sequence ...

This diagram shows how the number of pairs of rabbits over time forms the Fibonacci sequence. (Wikimedia Commons CC BY-SA 3.0)If you start with one pair, you soon get two pairs, three, five, eight ...