Fibonacci numbers have all sorts of amazing properties and links to many different kinds of mathematics. They start off with 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, and continue on from there.
Given the first two numbers of the sequence, 0 and 1, each succeeding number is simply the sum of the previous two numbers.
Technically, you can describe the sequence in the following way:
F0 = 0, F1 = 1, and for n greater than 1, Fn = Fn–1 + Fn–2.