Fibonacci sequence - position

I search something about recursionl, in particular Fibonacci sequence and I see this example:

var fibonacci_series = function (n) 
  if (n===1) 
    return [0, 1];
    var s = fibonacci_series(n - 1);
    s.push(s[s.length - 1] + s[s.length - 2]);
    console.log(s[s.length -1]); // add me
    console.log(s[s.length -2]); // add me
    return s;


and I don’t understand what means s[s.length -1] and s[s.length-2], I mean they represent the positions or other things.
If it presents the positions which one does it take?
Or there’s something that escapes me and I can’t see.
I ask if there is any resource that explains it in a simple step by step manner.

Not sure what you mean by “which one does it take”. s[s.length - 1] and s[s.length - 2] are the last and 2nd to last elements in the s array. They get added together to become the next element (via the push method) in the s array.

When I looked at that line of code, I think this:
If i get for example 3 as parameter of function,
executes the function, arrives at the variable s,that recall function ,but
instead having 3 now I have 2 (if I do n -1),then it goes on until it reaches 1.
Arrived at the line s.push(s[s.length - 1] + s[s.length - 2]);
I had thought it was making the sum and add element in s.
(I don’t know if I’m thinking well here)