(Ha-Mim. By the manifest Book.) means, plain and clear in both wording and meaning, because it was revealed in the language of the Arabs, which is the most eloquent language for communication among people. Allah says:
إِنَّا جَعَلْنَـهُ
(Verily, We have made it) meaning, revealed it,
قُرْءَاناً عَرَبِيّاً
(a Qur'an in Arabic) meaning, in the language of the Arabs, eloquent and clear;
لَعَلَّكُمْ تَعْقِلُونَ
(that you may be able to understand.) means, that you may understand it and ponder its meanings. This is like the Ayah:
This Surah is Makki, although Muqatil رحمۃ اللہ علیہ has said that the verse وَاسْأَلْ مَنْ أَرْسَلْنَا (43:45) is Madam, and according to another view, this Surah was revealed in Heaven during Lailat-ul-Mi` raj (the Night of Ascention) (Ruh u1-Ma’ ani) - Allah knows best.
Ha Meem- Only Allah knows its meaning.
وَالْكِتَابِ الْمُبِينِ (By the manifest Book - 43:2) The book in this verse refers to the Holy Qur'an. Whenever Allah swears by anything, it is usually an argument for the statement that follows. Swearing by the Qur'an in this verse is an indication that the Qur'an, by virtue of being a miracle, is a proof in itself of its being a Divine Book. To call it a 'manifest book' means that its subjects consisting of exhortations and advices are easily understandable; but as far as deduction of the precepts of 'Shari'ah' is concerned, it certainly is a difficult job which cannot be performed without complete capability of 'ijtihad'. This point has been clarified in Surah Alqamar, verse 17 وَلَقَدْ يَسَّرْنَا الْقُرْآنَ لِلذِّكْرِ فَهَلْ مِن مُّدَّكِرٍ (And indeed We have made the Qur'an easy for seeking advice. So, is there one to seek advice? - 54-17). Here, it has been stated that the Qur'an is easy for obtaining advice. Hence, it does not necessarily follow that ijtihad and inferring injunctions will be easy also; rather, it is proved through other evidences that full expertise in related subjects is a necessary condition for this exercise.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 4–7
The Original Book, or the Mother of Book (ummul kitab), refers to the preserved tablet (lawh mahfuz), which is with God. On this tablet, God Almighty has an imprint of the true religion, which He requires human beings to follow. This true religion, revealed to numerous prophets in various languages, was revealed to the Last of the Prophets in Arabic. Today, it is only this Arabic Quran which represents the true religion of God. It is now the responsibility of the bearers of the Quran to have it translated into every language and make it available to all the nations of the world, so that this religion may be understood by all other people, as it was understood by the Arabs. These sublime scriptures, being full of wisdom, are in themselves proof of their being the Book of God. The language of the Quran and its contents do justice to the Majesty of God with the utmost exactitude. Had the Quran been couched in the words of human beings, it would not have possessed the extraordinary glory which it possesses till today.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.