Allah, may He be exalted, says (interpretation of the meaning): “Blessed be He Who has placed in the heaven big stars, and has placed therein a great lamp (sun), and a moon giving light” [al-Furqaan 25:61].
And He says (interpretation of the meaning):
“And indeed, We have put the big stars in the heaven and We beautified it for the beholders”
“By the heaven holding the big stars”
The words “Blessed be He” mean: May He be exalted, Whose giving is plentiful and Whose blessings are abundant.
So the Lord, may He be exalted, deserves gratitude, veneration and praise for His blessings.
See: Tafseer Ibn Katheer, 6/92; al-Jaami‘ li Ahkaam al-Qur’aan, 7/223 and 13/1; Fath al-Qadeer, 3/683
“…Who has placed in the heaven big stars, and has placed therein a great lamp (sun), and a moon giving light”
“big stars”: Ibn ‘Abbaas, Mujaahid, al-Dahhaak, al-Hasan, Qataadah and al-Saddi stated that “burooj” [translated here as “big stars”] refers to stars.
Tafseer Ibn Katheer (8/363)
Shaykh al-Sa‘di (may Allah have mercy on him) said:
Allah says, describing His perfect power and His Mercy towards His creation:
“And indeed, We have put the big stars in the heaven” i.e., stars like towers, and great signs by which man may navigate in the darkness on land and sea, “and We beautified it for the beholders”[al-Hijr 15:16], for were it not for the stars, this wondrous sight in the heavens would not exist. This is something that calls those who gaze upon it to ponder it, seek its meaning and see it as evidence of its Creator. End quote.
Tafseer al-Sa‘di, 1/430
And it was said that burooj refers to the constellations. Al-Shawkaani (may Allah have mercy on him) said:
What is meant by burooj is the groups of stars, i.e., their constellations. And it was said that it refers to big stars, but the former is more correct. They are called burooj, which refers to lofty palaces, because they are like high built dwellings. The word burj is derived from tabarruj, which refers to an open display. End quote.
Fath al-Qadeer, 4/122
The word “a lamp” refers to the sun, and “a moon giving light” means it shines.
Ibn Katheer (may Allah have mercy on him) said:
“and has placed therein a great lamp” refers to the shining sun.
“and a moon giving light” means it shines with the light of another.
To sum up, what the verse means is: Exalted and venerated be Allah, Whose giving is plentiful, for His blessings upon His creation are evident. He is the One Who, by His grace, has adorned the lowest heaven with these great stars and planets, and the sun and the moon which give light by night and day. He has created that as part of His perfect blessings to His creation; His blessings are vast and His generosity and kindness are immense. So let them ponder the great signs that are above them, and the blessings that they enjoy, so that they might believe in their Lord and give thanks to Him.
And Allaah knows best.