When you see a sensitivity rating, it is always written as a factor of SPL output from a particular distance. This is a rating that tells you what the output of a speaker is when fed a 1kHz signal (sine wave) at exactly 1 Watt (2.83V) and measured on-axis from 1 meter away. This is a great number because it tells you how “efficiently” the speaker uses amplifier power. In a sense, you could call this how “loud” the speaker is, but that’s not entirely true since a less efficient speaker might be able to take more power before distorting.
To give you an idea of how much this matters, consider that the way we perceive something as being “twice as loud” means that it has increased by 10 dB SPL. If one speaker has a rating of 84 dB SPL @ 1W/m and another has a sensitivity of 94 dB SPL @ 1W/m then it will take twice the amplifier power to bring the first speaker to the same volume as the second. That’s an amazing difference that will mean a lot when you’re trying to fill a larger room or play back music at much louder volumes.