Sensitivity is a measurement of how much voltage is produced by a microphone for a specific measured input (1 Pascal). Sensitivity is measured in micro volts (uV) or millivolts (mV).
A typical rating for a dynamic microphone would be 1 mV. A typical rating for an Electret microphone would be 3.5 to 10 mV A typical rating for an Electret with amplifier would be 170 mV.
The higher the number the stronger the signal produced by the microphone.