Here Allah tells us about the Book which He revealed to His servant and Messenger Muhammad ﷺ.
وَأَنَّهُ
(And truly, this) refers to the Qur'an, which at the beginning of the Surah was described as
وَمَا يَأْتِيهِم مِّن ذِكْرٍ مِّنَ الرَّحْمَـنِ مُحْدَثٍ
(and never comes there unto them a Reminder as a recent revelation from the Most Gracious...) (26:5).
لَتَنزِيلُ رَبِّ الْعَـلَمِينَ
(is a revelation from the Lord of Al-`Alamin.) means, Allah has sent it down to you and revealed it to you.
نَزَلَ بِهِ الرُّوحُ الاٌّمِينُ
(Which the trustworthy Ruh has brought down.) This refers to Jibril, peace be upon him. This was the view of more than one of the Salaf: Ibn `Abbas, Muhammad bin Ka`b, Qatadah, `Atiyyah Al-`Awfi, As-Suddi, Ad-Dahhak, Az-Zuhri and Ibn Jurayj. This is an issue concerning which there is no dispute. Az-Zuhri said, "This is like the Ayah:
قُلْ مَن كَانَ عَدُوًّا لِّجِبْرِيلَ فَإِنَّهُ نَزَّلَهُ عَلَى قَلْبِكَ بِإِذْنِ اللَّهِ مُصَدِّقًا لِّمَا بَيْنَ يَدَيْهِ
(Say: "Whoever is an enemy to Jibril -- for indeed he has brought it down to your heart by Allah's permission, confirming what came before it...") (2:97).
عَلَى قَلْبِكَ
(Upon your heart) `O Muhammad, free from any contamination, with nothing added or taken away.'
لِتَكُونَ مِنَ الْمُنْذِرِينَ
(that you may be of the warners,) means, `so that you may warn people with it of the punishment of Allah for those who go against it and disbelieve in it, and so that you may give glad tidings with it to the believers who follow it.'
بِلِسَانٍ عَرَبِىٍّ مُّبِينٍ
(In the plain Arabic language.) meaning, `this Qur'an which We have revealed to you, We have revealed in perfect and eloquent Arabic, so that it may be quite clear, leaving no room for excuses and establishing clear proof, showing the straight path.'
وَإِنَّهُ لَفِى زُبُرِ الاٌّوَّلِينَ أَوَلَمْيَكُن لَّهُمْ ءَايَةً أَن يَعْلَمَهُ عُلَمَاءُ بَنِى إِسْرَءِيلَوَلَوْ نَزَّلْنَـهُ عَلَى بَعْضِ الاٌّعْجَمِينَ