Sensitivity is the measure of the loudspeaker's ability to convert electrical power into sound energy. It is quoted by the resulting SPL in dB in a distance of 1 meter then the speaker is powered by 1 watt. Typically it is measured with a test tone of 1kHz. Normally the measurement microphone is placed one meter away from the loudspeaker to measure the sound output. For larger systems the measurement will be made in a larger distance and the result is re-calculated for 1 meter.
Often the measurement is not made by '1 watt' but by feeding 2.83 volts into the speaker. With an internal impedance of 8 ohms, this will result also in a power rating of 1 watt. But if a loudspeaker has a slightly lower impedance (7 ohms or 6 ohms as it is normal for certain frequency ranges), the voltage of 2.83 volts will result in a higher power than one watt and this way the speaker will produce a higher sound output than really fed with '1 watt'. Sensitivity ratings should be carefully compared between different brands.