To be able to connect an electret microphone which produces weak audio signal, to an audio power amplifier or something like that, 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, for example, a power amplifier. 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.