The "Delivered Within" filter is for specifying "only send this message to an order if it was delivered within X amount of days".
For example, if you choose 7 days, then the message will only send to an order if the order was "delivered within" 7 days. If the "Delivered Within" filter is left blank, it does not matter how long it took for the order to get delivered to the customer and the message will send regardless.
Please note: This filter only applies to the Order Event of "Delivered." You will not be able to select this filter on any other Order Event.