Daylight, night time and twilight

The darkness meter:

The meter appears in the bottom left of the star map window. A red pointer shows the darkness of the sky based upon how deeply the sun has set below the local horizon.

All of these terms are explained below.

The 'Daylight' daily report:

Get a report on any day's sunrise, sunset and twilight times at your home location by selecting Daylight from the Daily report menu:

Twilight is that time before sunrise or after sunset when the sky is not totally dark.
Civil twilight occurs when the sun is 6° or less below the horizon. The sky is too light to see anything but the very brightest objects.
Nautical twilight begins and ends when the sun reaches 12° below the horizon. During this time the bright stars which were once used for navigation are clearly visible and the sea horizon can still be seen.
Astronomical twilight starts and ends when the sun is up to 18° below the horizon. In a clear sky stars of the 6th magnitude (the faintest objects visible to the naked eye) are just visible overhead.

A note about moonlight and seasons:
Despite what Stargaze's daylight report says or the 'darkness meter' shows, the sky can only be completely dark on moonless nights.

Depending upon your geographical latitude it is quite possible that, during the summer months, twilight can last all night. This example (taken in July for the UK) is for a day when the sky will never be totally dark...