(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:
A Preacher must not give up his preaching due to disappointment
أَفَنَضْرِبُ عَنكُمُ الذِّكْرَ صَفْحًا أَن كُنتُمْ قَوْمًا مُّسْرِفِينَ (Shall we remove the advice away from you because you are a transgressing people? - 43:5) The meaning is that We will not give up advising you through the Qur'an, no matter how rebellious and disobedient you may become. This tells us that the person who is engaged in preaching and inviting others towards Islam should carry the message to everybody; he must not give up preaching to some people or any group simply because they are non-believers, highly irreligious, sinners and transgressors.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 8–11
Today there are countless people in the world who speak of previous prophets with reverence. In view of this, it appears very strange that these very prophets (including the Prophet Muhammad) were ridiculed and mocked by their contemporaries. The reason for this is not that the people of those days were savage and the people of today are civilized. It is simply due to a difference in the times. Today, after the lapse of centuries, every prophet is surrounded by an aura of historical glory. So, every superficial admirer of appearances recognises the prophet. But, to his contemporaries, the prophet appeared to be an ordinary person. At that time, in order to discover and recognise the reality of the prophetic position of a prophet, a deep insight was required to discover the reality and, undoubtedly, such insight has always been very rare in this world. However badly a preacher’s addressees may treat him, he unilaterally exercises patience and carries on the work of his mission, until the day comes when God’s will is made manifest on what future course he should take.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.