Understanding for loops

@sophielevens asked me this question about understanding for loops. The question is from Free Code Camp.

**Question**
If you have a multi-dimensional array, you can use the same logic as the prior waypoint to loop through both the array and any sub-arrays. Here is an example:
```
var arr = [
[1,2], [3,4], [5,6]
];
for (var i=0; i < arr.length; i++) {
for (var j=0; j < arr[i].length; j++) {
console.log(arr[i][j]);
}
}
```
This outputs each sub-element in arr one at a time. Note that for the inner loop, we are checking the .length of arr[i], since arr[i] is itself an array.
**Instructions**
Modify function multiplyAll so that it multiplies the product variable by each number in the sub-arrays of arrand the answer is this
```
function multiplyAll(arr) {
var product = 1;
// Only change code below this line
for (var i=0; i < arr.length; i++) {
for (var j=0; j < arr[i].length; j++) {
product *= arr[i][j];
}
}
// Only change code above this line
return product;
}
// Modify values below to test your code
multiplyAll([[1,2],[3,4],[5,6,7]]);
```

**Her questions were:**
1. Where on earth did [i] and [j] come from? Have googled it and can't find out what they mean
2. Also, (i++) is apparently the same as (i = i + 1) why do we want to add 1 to i?

issue では、リポジトリのファイルもテストも指定されていません。提供された multiplyAll の例から始めて、その周辺の JavaScript 学習教材を確認してください。教材が i と j が何を表すのか、ループがそれらをインクリメントする理由、ネストされた配列をどのように走査するのかを明確に説明できていれば完了です。

索引モデルが issue の本文から書いたものです。