The literal meaning is Consider the star when it sets.
The majority of commentators of the Holy Quran admit that the term Najm derived from the verb najama meaning it appeared began, ensued or proceeded. It denotes unfolding of something which appears gradually, as by instalments.
Thus the parts of the Holy Quran gradually revealed are referred to nujum of the Holy Quran. This explanation was given by H Ibn Abbas RA, quoted by Tabari, and same interpretation was justified by others like Raghib, Zamakhshari, Razi, Baydawi, Ibn Khatir etc.
Verse 56:75 is given as example to understand this verse i.e. gradual step by step revelation of the Holy Quran.
فَلَا أُقْسِمُ بِمَوَاقِعِ النُّجُومِ (
56:75)
56:75 NAY, I call to witness the coming-down in parts [of this Quran]
Furthermore, if you look down at the following verses, it clarifies that the Holy Quran was imparted to Prophet SAW by a mighty in power, i.e. Gabriel As and that the Holy Prophet SAW was not a mad man (nauzobillah) a charge which was leveled at him SAW by the Kafroon.
The Holy Quran described two incidents when Prophet SAW has saw Gabriel As in his true manifestation. Described in this Surah.
First one described like him As appearing to cover the whole visible Horizon from the view and standing physical position of Prophet SAW.
Second sighting is described in
وَلَقَدْ رَآهُ نَزْلَةً أُخْرَى (53:13)
53:13 And, indeed, he saw him a second time
53:14 by the lote-tree of the farthest limit,
عِندَ سِدْرَةِ الْمُنْتَهَى (53:14)
At the time of Miraj, at Sidarath ul Muntha, farthest limit , where Gabriel As could go no further.
These are only two incidents when Prophet SAW saw Gabriel As in his true form and manifestation according to the Holy Quran. And Allah SWT knows the best.