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.