Reading the graph
The graph shows how your system sounds in your room, frequency by frequency, at the spot where you measured.
Frequencies and levels
Across the bottom are the frequencies, in hertz (Hz): the deepest bass on the left, higher notes on the right. Up the side is how loud each frequency reaches the microphone, in relative dB. The amber curve is the subwoofer, the cyan one the speakers.
Zero is not an absolute level: the iPhone microphone is not calibrated, so what matters is the shape of the curves and the differences between them, not the numbers themselves.
Peaks and dips
A curve goes up and down: that's normal in every room. Walls reflect bass, and in some places the waves add up while in others they cancel.
A peak sounds like boom, one note ringing louder than the others. A dip is a range you barely hear, even with the volume up. Slope marks the strongest one as Highlighted peak or Highlighted dip.
Room peaks and dips change a lot when you move the sub or the listening position, even slightly. That's why the Suggestions may ask you to try another position.
Below 40 Hz
At the lowest frequencies, the iPhone microphone and room noise make readings less reliable. Slope still shows the curve, but marks that area.
Before adjusting anything for a peak or dip that sits entirely below 40 Hz, measure again and check that it shows up again.
Don't chase a flat line
No room gives a perfectly flat curve, and you don't need one. Ripples of a few dB are normal: look at the biggest peaks and dips, and compare before and after each adjustment.
Comparing two measurements
In the Library you can open a measurement and compare it side by side with another. The comparison makes sense when both were taken at the same spot, with the same volume and the same audio output: that way you see only the effect of what you changed.
Change one thing at a time, then measure again. If you change several things at once, you won't know which one made the difference.