When you reference an index in an array, in English that would read "Get me the item at this position in the array".
If we want to think of an analogy for an array, let's think of it as a set of P.O. boxes at your local post office, you're the postmaster that's delivering mail. You might have mail for PO Box 123, you have an overview of all the PO Boxes and you can see exactly where #123 is located, so you go straight to it. Next up is mail for #132, again - you know exactly where it is - you don't need to count all the boxes up until the correct number.
In practice, let's take your array of months for example and you call aryMonths you will get the item at index 4 (which would be May, because arrays are 0-indexed).
When you return the value in your return statement the following happens:
return aryMonths[NUM-1]; //this is what you see
//let's say you have MonthAsString(4)
//this ends up being:
//what actually happens is:
return aryMonths; //It's important to note that the calculation happens first.
just to add to this a little,
if you wanted to know the last item in an array you would subtract 1 from the array length.
in this case, the array length is 12, which means the last index is 11. Handy if you have a
dynamically populated array whose length you don't know
aryMonths[ aryMonths.length - 1];
So in short. someArray[n] will always directly reference an index in an array, not iterate over all items until it reaches the correct one.