It means that the light of the sun and the moon complement each other, or the sun and the moon appear at the same time.
The sun and the moon shine together is a natural phenomenon caused by the different rotation periods of the earth around the sun and the moon around the earth. Certain conditions are required for direct observation of the sun and moon astronomical phenomenon with the naked eye. Generally, it can only be seen in broad daylight during the 21st to 23rd lunar calendar in the autumn months when the sky is clear and visibility is good.
Extended information
Principle of the sun and the moon shining at the same time
The sun and the moon shining at the same time is just a natural phenomenon. The phenomenon of the sun and the moon shining at the same time requires that the sun and the moon shine at the same time. above the horizon. Theoretically, as long as the atmospheric conditions are relatively good, this phenomenon may occur except on the fifteenth day of the lunar calendar. But there is a second condition, which is that the limit magnitude of the sky (the faintest celestial object that can be seen) cannot be too low, so generally speaking, the brightness of the sky easily meets this condition when the sun is rising or about to set (even if is a city).
So in the first half of the lunar calendar (after the fourth day of the lunar month), you can see the moon phases when the sun sets. From west to east, you can also see the moon phases in the second half of the lunar calendar (before the twenty-seventh day) after the sun rises. to the moon, and from the western sky to the eastern sky.