By the dawn and the ten nights, and by the end and the odd, and by the night when it passes;
Is every one of them not an oath (to God), for the one who understands?
This set of verses – just to contextualise this page briefly, as scholars have and will explain this better than I do – describes the regularity of time. The consistent passing of the day and night, and the day/night imagery, are reflections of God and the Muslim calendar, and their core values. The whole point of this surah (not just this quote) is to discuss the realness of afterlife, and the consequences for believers/sinners.