The Fibonacci Sequence is the series of numbers:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

The next number is found by adding up the two numbers before it.

The 2 is found by adding the two numbers before it (1+1)

Similarly, the 3 is found by adding the two numbers before it (1+2),

And the 5 is (2+3),

and so on!

More at:

https://www.mathsisfun.com/numbers/fibonacci-sequence.html