Harry potter?

"This is magic at its deepest, its most impenetrable, Harry. But trust me...the time may come when you will be very glad you saved Pettigrew's" This has been said by Dumbledore on HP-PoA. I'm just wondering if this explained on the last book. 'Cause I haven't read the last installment yet. All I can do is reread the other books and it so happened I come by with this. Any response may do..