It's purpose is to isolate The USB +5vdc from the on-board +5vdc voltage regulator for the case when there is both an external DC power source connected to the board and also plugged into a PC USB port at the same time. It's called the 'auto-voltage selector' circuit. It's never a good engineering practice to have two voltage regulators 'hardwired' together so this circuit is designed to prevent that from happening. The T1 mosfet is the active switch element and the op-amp is wired as a simple comparator to drive the mosfet on or off. The presence of a Vin voltage above a minimum valid value cause the current path from the USB power to be cut off.
Lefty