A sensitivity rating tells you how effectively a speaker converts power (watts) into volume (decibels). The higher the rating, the louder your speakers will play with a given amount of amplifier power. Sensitivity is often measured by driving a speaker with one watt and measuring the loudness in decibels at one meter.
The chart below illustrates that a few dB in sensitivity can make a big difference:
Speaker Sensitivity rating Power needed to produce a given volume
Speaker A 85 dB 100 watts
Speaker B 88 dB 50 watts
Speaker C 91 dB 25 watts
A speaker with a sensitivity rating that's 3 dB higher than another speaker's only needs half as much power to deliver the same amount of sound.