Hi,
I think you meant C20 and R28 right?
Those two are used to reduce the voltage to the circuit without requiring a transformer. That kind of power supply is often referred to as an "offline" power supply. The capacitor reduces the AC level without dissipating too much power while a single resistor would dissipate much more power and get hot. The resistor R20 limits the input surge current when the power supply is first plugged into the main AC supply. Without that resistor the surge current through the cap could be very high.
The resistor value is chosen to reduce the surge yet not dissipate too much power during normal operation. The capacitor value is chosen to allow enough current to pass so the circuit being powered gets enough voltage but not too much.