Electret microphone board strange offset using Arduino
An electret microphone is like a resistor which changes its value according to the sound.

To use it you must form a voltage divider with it, as if it were half of a potentiometer.

Thus you add a resistor to +5V (which is already on the board).  This means that when it is silent the voltage you get is the ratio of the resistor and the resistance of the microphone.

If the resistance has been chosen correctly then the output you get should be roughly half the supply voltage when silent. Any sound will cause that output to swing around that "zero" point.

It's just like a potentiometer.  When silent the potentiometer is in the central point. When you make noise you're moving the potentiometer in both directions.

To get rough values you should first get the "silent" reading from analogRead. You can then subtract that value from all subsequent readings. 

Note that the results will be non-linear. To do it properly you should calculate the resistance of the microphone by using the supply voltage, the "upper" resistor value, and the sampled voltage to calculate the resistance value like you would for any other resistive sensor (such as an LDR or force sensitive resistor).