Actually, 70 and 80db is pretty loud.
The absolute limit of what the human ear can hear depends upon many factors, of which the most important is the ear itself. However, as the rating system goes, they use "zero" as the baseline for sound pressure levels not detectable by the normal human ear.

Can't recall the exact number but longer term exposer to sound over 80 or 85, certainly 90db, can be damaging. It's fine for short bursts like the quick loud scenes in movies. I think 120 is the safe limit for short bursts of loudness.

They basically use the graphs to give you an indication of how well the speaker does at consistently producing all the different frequencies of sound from lows to highs. You can see if a speaker has trouble producing the lower, bass sounds or how it performs with the high notes and midrange. A perfectly healthy, young human ear can hear frequencies from as low 20 Hz up to highs of 20 kHz

I'm sure more detail will follow but basically, a flatter graph tells you that the speaker is good at producing all of the frequencies at very close to the same sound levels. If there is a big dip anywhere, it means that you will not hear that frequency very well from that speaker. An upwards spike is equally bad. This changes how accurately you hear the sound as it's over or under emphasizing certain frequencies.

It is normal for a speaker to start to 'fade' as it hits the bottom end of the lows and up at the extreme highs but by comparing where it starts to take a nose dive, you can compare it to other speakers and see who produces better bass and so on.

Hope that answers your question.

With great power comes Awesome irresponsibility.