First I thought it's some sort of over-voltage protection but right after this, there's Zener diode doing the same?
That PMOS is being used for reverse polarity protection. I see the net DC_5V_IN is connected before the PMOS so it is not meant to protect reverse polarity from the DC barrel jack J2. That makes me think it is there to prevent current from being pushed to the 5V DC bus from the "wall" side of the circuit.
It looks like R1 and R2 are simply jumpers (0 ohm) that if you added to the circuit would bypass the PMOS altogether.
DNP may mean "Do Not Populate".