The First Eleven. Part 9, Page 2. The sensitivity tells you how loud a sound you'll get out of the speaker when you put a given amount of electrical power in. To understand this value you need to know what the decibel means.
Decibels are a logarithmic way of representing relative power levels.
In fact, the basic unit here is the Bel , although you almost never see it mentioned. A deci-bel is one tenth of a Bel. These are used to specify power levels compared to a specific reference level. This is regarded as about the faintest sound humans can hear. Hence a sound level in dBa indicates how loud something sounds rather than saying how much power is around at inaudible frequencies.