When you set your chart to use a Alt-Az coordinate system together with the usage of a automatic background colour, the sky background will display a lighter colour, related to the sort of twilight.
Astronomical dusk is the period of time during which the center of the sun is lower than 18º degrees of arc below the horizon for your observation position. For those periods, SkyChart will usually display a dark background, except when the Moon is above the horizon.
The last columns of the table show the begin and end of this dark period including for the Moon presence. The Moon illumination fraction is also indicated for a better estimation of how bad it will be.