"Nay, I swear by the twilight," meaning: the redness when the sun sets, between the time of Maghrib and Isha.
﴿فَلَا أقسم بالشفق﴾ يَعْنِي: الْحُمْرَةَ إِذَا غَابَتِ الشَّمْسُ مَا بَين الْمغرب وَالْعشَاء
