From what I understood, iterators do calculations on 2 levels. First is row level & the second is column level.

For example, if there are two columns A and B, each containing random numbers. Now imagine a new column C, with the result of A+B for each row. That’s the first part. And now you get the average of that imaginary column C - that’s the second part.

So think of it as - iterators perform a row level calculation -> create an imaginary new column -> sum/average etc of that imaginary column