Translate formula into a sentence

This formula is to return all the data that only in oldexchange.
Index() function returns the ?th cell in the range selected, in this formula, oldexchange. ? Will change according to the value returned by small() function.
small() function returns the ?th smallest row number. ? Will changed by the rows() function when you copy this formula down.
if() function to return an array of row numbers of all the data only in old range.
* row number is compare to cell A3, not A1

*since this is an array formula, remember to press shift+ctrl+enter instead of enter after you modified it.

Hope I can have made myself understandable