Electret microphones require a power supply to operate. This is provided as a DC offset imposed over the signal connection.
The resistor is to limit the current of that power - typical value is around 2K? to 3K?.
The capacitor is used to block that DC offset and stop it getting in to your circuitry. The value chosen depends on both the frequency response you want and the impedence of your input. Typically I use a 22µF capacitor as it gives good enough response. A lower value capacitor will reduce low frequency response.
Think of the electret microphone as a variable resistor that changes with the pressure gradient. As SPLs increase, the resistance increases, drawing more current across it. As SPLs decrease, the resistance decreases reducing the pull-down current. Thus RL and the electret microphone are just a potential divider where RL sets the current drawn from the supply rail. For battery circuits i'd recommend increases RL as much as you can.
