The higher the sensitivity rating the more volume you get when compared to a loudspeaker with a lower rating. Below 84dB is considered poor while 92dB and above is considered excellent.
Sensitivity is often confused with efficiency, but it is not exactly the same thing. Efficiency measures the ratio of power converted to sound energy compared to heat energy. Sensitivity measures the amount of SPL (Sound Pressure Level) a loudspeaker produces for a given amount of power. The specification is typically derived in an anechoic chamber with 1 Watt of power at 1kHz with a microphone placed 1 meter away).
Rule of Thumb
With each 3db gain in volume, output power is doubled. If your speaker/100-Watt amp combo plays at 87dB and you want to play at 90dB you will need 200 Watts of power, and so on.