Tafsir of Ash-Shams 91:3

Surah Ash-Shams (The Sun): Verse 3

وَالنَّهَارِ إِذَا جَلَّاهَا (And [by] the day as it reveals her [the sun/light])

The meaning of tajliyyah (جلاها) is manifestation and unveiling.

Regarding the pronoun (ḍamīr) in jalāhā (reveals her), there are two views:

First View (Al-Zajjaj's Opinion): The pronoun refers back to the Sun (الشمس). This is because the daytime (an-nahār) is essentially the light of the Sun. Therefore, the clearer and more manifest the daytime is, the clearer and more manifest the Sun is, because the strength and perfection of the effect indicate the strength of the cause (the Sun). Thus, the daytime brings out and reveals the Sun, similar to the verse: {None can reveal it at its appointed time except Him} (referring to the Hour/Resurrection), meaning none can bring it forth.

Second View (The Majority Opinion): The pronoun refers back to the Darkness (الظلمة), or the World (الدنيا), or the Earth (الأرض). Even if these nouns were not explicitly mentioned, it is permissible in Arabic usage. For instance, one might say, "It has become cold" (أصبحت باردة) meaning the morning (الغداة), or "She was sent" (أرسلت) meaning the sky (السماء).

وَاللَّيْلِ إِذَا يَغْشَاهَا (And [by] the night as it covers her)