Powering condenser mic while getting the signal
That resistor and the microphone form a voltage divider.  This means that the output voltage of these two devices will not be centered around 0V (it will have a DC offset somewhere between 0V and 9V).  The 0.1uF capacitor is a DC blocking capacitor, and the voltage at the output of that capacitor will be centered around 0V (ground) because of the output is biased by the 100k resistor.

If you build the circuit and examine the signals with an oscilloscope all of this will become obvious.  

You could also simulate this using LTSpice or other simulation program.  If you want to simulate this, replace the microphone and resistor with a voltage/signal source with a DC offset of 2V generating a sine wave with an amplitude of 10mV or so (op-amp has a gain of 100, you should get about 1V out).