diode and resistor in parallel connected to voltage source
The 10k resistor is a pull-up, makeing sure that the MOSFET is turned on by default.

The port on the right is probably a connector and pin 3 is an open collector which allows you to turn off the MOSFET by pulling it low. 

The diode is a clamp diode to make sure the gate voltage doesn't exceed 5 V, probably due to miller effect though the MOSFET. At small signal levels, this is neglectable but perhaps the desiner wanted protection against ESD/excessive drive voltage from the outside. You'll see them commonly even when not fully needed. 

That MOSFET in turn is an inverter and overall it looks like a 5 V TTL logic to 3.3 V translator which connects via a connector and assumes open-collector output on the sender. Output to your 3.3 V system is the middle wire on the left, the source of the MOSFET.