The Fibonacci series is a sequence of numbers formed by adding two sequential numbers to generate a third. The first two numbers are 0 (zero) and then 1.  The third number in the sequence is "0+1" and so also 1. The fourth number in the sequence is the sum of the previous two: "1+1", and so it is 2. The fifth is the sum of "1+2" and therefore 3.  Then "2+3" is 5, the next number. And so on, the sequence running:

   0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, . . . etc.

(If you are not sure you understand, see that the next-to-last number I have written in this case is 89 -- the sum of the two numbers just previous to it, 34 and 55.)

If you lay out a structure that expresses the Fibonacci series in space, it begins to form a spiral. Here is a common example of how that looks if the shapes are each square and the sides of the squares are the Fibonacci values. The structure is growing outward from the center. The size of the next square will be 13 units (5 + 8) on each side.