六等星の夜(Night of the magnitude six star) is the ending track of the anime series No. 6.

The song evokes good memories and expresses the sadness of a farewell, but mixed with the hope of meeting that person again. (Related to No.6)

六等星 refers to the stars of apparent magnitude 6.
In astronomy, the magnitude measures the brightness of a celestial body.
The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values.

Magnitude six stars are usually the limit human naked eyes can observe. But, only in a very dark sky and in places without light pollution.
So, these nights are the darkest, but also lighten by the dimly shining stars which represent the hope that always exists.