Well to understand you have to understand that the first two numbers in the fibonacci series are 0 and 1, you need those numbers to begin the sequence, every sequence starts somewhere, this return 0; and return 1; literally gives you the integers 0 and 1 for you to get the other numbers, with those two numbers you are capable of getting the third number in the sequence 0+1 = 1, with that you can get the fourth one 1+1 = 2 and so on, but without knowing they start at 0 or 1, the result is undefined