The Surah has been so designated after the word ash-shams with which it opens.
The subject matter and the style show that this Surah too was revealed in the earliest period at Makkah at a stage when opposition to the Holy Prophet (upon whom be Allah's peace) had grown very strong and intense.
Its theme is to distinguish the good from the evil and to warn the people, who were refusing to understand this distinction and insisting on following the evil way, of the evil end.
In view of the subject matter this Surah consists of two parts. The first part consists of vv. 1-10, and the second of vv. 11-15. The first part deals with three things:
That just as the sun and the moon, the day and the night, the earth and the sky, are different from each other and contradictory in their effects and results, so are the good and the evil different front each other and contradictory in their effects and results; they are neither alike in their outward appearance nor can they be alike in their results.
That Allah after giving the human self powers of the body, sense and mind has not left it uninformed in the world, but has instilled into his unconscious by means of a natural inspiration the distinction between good and evil, right and wrong, and the sense of the good to be good and of the evil to be evil.
That the future of man depends on how by using the powers of discrimination, will and judgment that Allah has endowed him with, he develops the good and suppresses the evil tendencies of the self. If he develops the good inclination and frees his self of the evil inclinations, he will attain to eternal success, and if, on the contrary, he suppresses the good and promotes the evil, he will meet with disappointment and failure.
In the second part citing the historical precedent of the people of Thamud the significance of Prophethood has been brought out. A Messenger is raised in the world, because the inspirational knowledge of good and evil that Allah has placed in human nature, is by itself not enough for the guidance of man, but on account of his failure to understand it fully man has been proposing wrong criteria and theories of good and evil and thus going astray. That is why Allah sent down clear and definite Revelation to the Prophets (peace be upon them) to augment man's natural inspiration so that they may expound to the people as to what is good and what is evil. Likewise, the Prophet Salih (peace be upon him) was sent to the people of Thamud, but the people overwhelmed by the evil of their self, had become so rebellious that they rejected him. And when he presented before them the miracle of the she camel, as demanded by themselves, the most wretched one of them, in spite of his warning, hamstrung it, in accordance with the will and desire of the people. Consequently, the entire tribe was overtaken by a disaster.
While narrating this story of the Thamud nowhere in the Surah has it been said "O people of Quraish, if you rejected your Prophet, Muhammad (upon whom be Allah's peace and blessings), as the Thamud had rejected theirs, you too would meet with the same fate as they met." The conditions at that time in Makkah were similar to those that had been created by the wicked among the people of Thamud against the Prophet Salih (peace be upon him). Therefore, the narration of this story in those conditions was by itself enough to suggest to the people of Makkah how precisely this historical precedent applied to them.
Source: Sayyid Abul Ala Maududi - Tafhim al-Qur'an - The Meaning of the Quran