To be able connecting an electret microphone which produces weak audio signal to an audio power amplifier or something, a so-called preamplifier circuit should be used.
This circuit amplifies the microphone's weak output voltage to a level of voltage which can drive a power amplifier for example. It is also possible to use this circuit to sample the audio from an electric microphone by an Analog to Digital converter.
By adjusting the potentiometer P1 the gain of this preapmlifier will change. Try to get the best adjustment which is a trade-off between the output amplitiude and the level of noise.