It means: the sun sets and disappears into the endless star field, and the sky is dark. It's night before you know it, looking at the surrounding scenery under the afterglow of the sunset, just like autumn. When I saw the sunrise in the morning, it was another sunny day. Everything is the same as before, and the country is safe.
This couplet is associated with the rivers and mountains of the motherland from the scene of sunset and sunrise, expressing a patriotic feeling of worrying about the country and the people.