Explain definition of fibonacci

Where an input value gives a trivial result, it is returned directly, otherwise the function calls itself, passing a changed version of the input values. Care must be taken to define functions which will not call themselves indefinitely, otherwise the program will never finish.

The  definition  of  fib  is  interesting,  because  it  calls  itself  twice  when  recursion  is  used. Consider the effect on program performance of such a function of a moderate size number