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?