If there is no j48 jumper and I plugged the barrel power what will happen will it power the nano, will it have no power or will it destroy the nano?
As long as the power is the correct voltage, then having J48 or not will never harm it. You have to be careful though, because some dev kits have the same barrel connector and run 19.5V, while your Nano might be looking for 5V.
This jumper is used to disable the USB power option. Removal of this jumper means you can power the board by DC-jack or USB port.