The black area down the centre of the diagram represents night, the light blue areas, day. The three dark stripes of blue on either side of the diagram represent Civil (dark blue), Nautical (darker blue) and Astronomical (darkest blue) Twilight.
The moon is depicted by the cream coloured horizontal lines, the thickness of which indicates the phase of the moon (thick = full moon, no line = new moon). The lines only appear when the moon is above the horizon at your location.
The diagram defaults to the nighttime view where the single vertical line down the centre of the diagram represents midnight. The dates down the side of the diagram therefore start at this midnight position rather than the left hand side of the diagram. So, day 1 comprises the right half of the line marked as '1' and the left half of the day marked as '2'. This may take a bit of getting used to, but just remember that each line on the diagram covers from midday on one day to midday on the next.
The diagram can also be switched into daytime mode. In this mode, each line on the diagram represents a day running from midnight to midnight, but each complete line now represents a single contiguous day. The line down the middle of the diagram represents mid-day in this mode.
The diagram is also capable of displaying an overlay grid with divisions for each hour in the day (labeled at the top of the diagram). The hour numbering will depend on whether daytime or nighttime view is in effect. The hour grid is there just to give some idea of when the sun rises/sets, twilight start and end times and moon rise/set times.
The diagram, as it appears on screen (in day/night mode, with or without grid) can also be printed out.