Using recursion to return the sum of the first n elements of an array arr

I'm trying to learn some JavaScript from freecodecamp.org and I stumbled upon a recursion problem which I can't wrap my head around:

Instructions: Write a recursive function, sum(arr, n), that returns the sum of the first n elements of an array arr.

function sum(arr, n) {
 if(n <= 0) {
    return 0;
 } else {
    return sum(arr, n - 1) + arr[n - 1];
}
}

I understand the second part: arr[n -1] adding the item value

But I don't understand what the first part(sum(arr, n-1)) does, and how the arr parameter reacts.

Can anyone help?

Any help is appreciated!