This refers to how Allah sends revelation. Sometimes He casts something into the heart of the Prophet , and he has no doubt that it is from Allah, as it was reported in Sahih Ibn Hibban that the Messenger of Allah ﷺ said:
«إِنَّ رُوْحَ الْقُدُسِ نَفَثَ فِي رُوعِي أَنَّ نَفْسًا لَنْ تَمُوتَ حَتْى تَسْتَكْمِلَ رِزْقَهَا وَأَجَلَهَا، فَاتَّقُوا اللهَ وَأَجْمِلُوا فِي الطَّلَب»
(Ar-Ruh Al-Qudus i.e., Jibril breathed into my heart that no soul will die until its allotted provision and time have expired, so have Taqwa of Allah and keep seeking in a good (and lawful) way.)
أَوْ مِن وَرَآءِ حِجَابٍ
(or from behind a veil) -- as He spoke to Musa, peace be upon him. He asked to see Him after He had spoken to him, but this was not granted to him. In the Sahih, it recorded that the Messenger of Allah ﷺ said to Jabir bin `Abdullah, may Allah be pleased with him:
«مَا كَلَّمَ اللهُ أَحَدًا إِلَّا مِنْ وَرَاءِ حِجَابٍ، وَإِنَّهُ كَلَّمَ أَبَاكَ كِفَاحًا»
(Allah never speaks to anyone except from behind a veil, but He spoke to your father directly.) This is how it was stated in the Hadith. He Jabir's father was killed on the day of Uhud, but this refers to the realm of Al-Barzakh, whereas the Ayah speaks of this earthly realm.
أَوْ يُرْسِلَ رَسُولاً فَيُوحِىَ بِإِذْنِهِ مَا يَشَآءُ
(or (that) He sends a Messenger to reveal what He wills by His leave.) as Jibril, peace be upon him, and other angels came down to the Prophets, peace be upon them.
إِنَّهُ عَلِىٌّ حَكِيمٌ
(Verily, He is Most High, Most Wise.) He is Most High, All-Knowing, Most Wise.
وَكَذَلِكَ أَوْحَيْنَآ إِلَيْكَ رُوحاً مِّنْ أَمْرِنَا
(And thus We have sent to you Ruh of Our command.) means, the Qur'an.
مَا كُنتَ تَدْرِى مَا الْكِتَـبُ وَلاَ الإِيمَـنُ
(You knew not what is the Book, nor what is Faith.) means, `in the details which were given to you in the Qur'an.'
وَلَـكِن جَعَلْنَـهُ
(But We have made it) means, the Qur'an,
نُوراً نَّهْدِى بِهِ مَن نَّشَآءُ مِنْ عِبَادِنَا
(a light wherewith We guide whosoever of Our servants We will. ) This is like the Ayah:
قُلْ هُوَ لِلَّذِينَ ءَامَنُواْ هُدًى وَشِفَآءٌ وَالَّذِينَ لاَ يُؤْمِنُونَ فِى ءَاذَانِهِمْ وَقْرٌ وَهُوَ عَلَيْهِمْ عَمًى
(Say: "It is for those who believe, a guide and a healing. And as for those who disbelieve, there is heaviness (deafness) in their ears, and it (the Qur'an) is blindness for them.") (41:44).
وَأَنَّكَ
(And verily, you) means, `O Muhammad,'
لَتَهْدِى إِلَى صِرَطٍ مُّسْتَقِيمٍ
(are indeed guiding to a straight path.) means, the correct behavior. Then Allah explains this further by saying:
صِرَطِ اللَّهِ
(The path of Allah) meaning, His Laws which He enjoins.
الَّذِى لَهُ مَا فِى السَّمَـوَتِ وَمَا فِى الاٌّرْضِ
(to Whom belongs all that is in the heavens and all that is on the earth.) means, their Lord and Sovereign, the One Who is controlling and ruling them, Whose decree cannot be overturned.
أَلاَ إِلَى اللَّهِ تَصِيرُ الاٍّمُورُ
(Verily, all matters at the end go to Allah.) means, all matters come back to Him and He issues judgement concerning them. Glorified and exalted be He far above all that the evildoers and deniers say. This is the end of the Tafsir of Surat Ash-Shura