circuit analysis of a PIR circuit
The resistors and cap work like a filter or bias voltage for the PIR sensor used in that module. Note that the BIS0001 (generic PIR sensor IC) does not require it, and many modules don't have it. It really depends on how good the sensor is.

The diode is just for reverse voltage input protection. It also drops the input voltage by ~0.7 volts, so the regulator drops less as well.