(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:
وَقِيلِهِ يَا رَبِّ إِنَّ هَـٰؤُلَاءِ قَوْمٌ لَّا يُؤْمِنُونَ (And [ Allah has the knowledge ] of His prophet's saying, "0 my Lord, these are a people who do not believe." 43:88) The point made by this sentence is that there are very serious reasons for Allah's wrath descending upon these infidels. On the one hand, there are their severe crimes themselves, and on the other when the Messenger ﷺ ، who has been sent as a "blessing for the world" and "interceder for the sinners", himself complains against them and says that these people do not believe, despite repeated good counsels given to them, it indicates that they must have agonized the Holy Prophet ﷺ to an extreme extent, otherwise "the blessing for the worlds" would not have made such a plaint merely on an ordinary hurt.
It should be noted that the present verse has been construed in different ways. The above translation is based on the construction adopted by Maulana Ashraf ` Ali Thanawi (رح) . Various other constructions are given in Ruh-ul-Ma` ani.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 85–88
Heaven and earth are continuously at work in perfect harmony. Perfect unity of wisdom and unity of knowledge are found in them. This proves that there is only one God Who is alone running the systems of both heaven and earth. The universe introduces us to God’s unlimited power and at the same time His unlimited mercy. These facts call for man to fear God most of all, and at the same time, entertain the greatest hopes of Him. Those who prove to have this consciousness and this character in this world are the only individuals upon whom—when they reach God—He will shower His infinite Mercy.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.