The ±3 dB is the range where the speaker is considered audibly flat. Differences off less than 3 dB are hard to distinguish when at different frequencies.
The +3 dB to -9 dB covers the usable range of the speaker. The -9 is almost always on the bass end of things. While you'd be able to notice less output in the lower frequencies, there's still some usable output even 9 dB down.
Impedance is a lot more technical. It refers to how easily alternating current flows through the speaker at different frequencies. It's enough to say that you don't really need to worry about it, just get a good quality amp, and everything will be fine.
X-Over (or crossover) points are the points where each driver (or pair of drivers in the case of the M80) blends into the next. From the woofers to the midrange at 160 Hz, and the mids to the tweeters at 2.3 kHz. This doesn't change the performance of the speaker at all, it is just up to the designer to properly select these points for the drivers (and enclosure) being used.
Really, the only thing that tells you about the performance of the speaker is the frequency response range. 20 kHz is enough on the upper end, and as long as the bottom goes low enough for your taste, or can at least get down to where the sub is going to pick up (80 Hz typical), then it's good enough.
