Exercise 7:recursion

[caoxiaoshuai1]

function reduce(arr, fn, initial){
    return (function reduceOne(index, value){
        if(index > arr.length - 1) return value
        return reduceOne(index+1,fn(value,arr[index],index,arr))
    })(0,initial)
}

module.exports = reduce;

What's the code means, I can't understand how it works. Especially for line 4.