Those devices are not normally on the Nano. It appears like you are reverse engineering a project. I would guess they are MOVs See what the other end is connected to, that will give you a hint. If it works measure the voltages etc.
A MOV directly on a pin would have been a designer mistake.No protection at all when the Arduino is off, and it's knee point would not be sharp enough to protect when on.Leo..
What else should a MOV on a pin be used for.Over-voltage protection is the only thing I can think off.Generally speaking, a MOV, or a zener, or a TVS diode can't fully protect the pin. Only clamping diodes can.Don't forget that max pin voltage is not 5volt. It's VCC + 0.5volt (and GND - 0.5volt).When the Arduino is off, VCC is zero volt.Leo..
I only have an image, no board in hand.