In the Name of Allah, the Most Gracious, the Most Merciful.
(Ha Mim. A revelation from the Most Gracious, the Most Merciful.) means, the Qur'an is revealed from the Most Gracious, Most Merciful. This is like the Ayat:
(Say Ruh Al-Qudus (Jibril) has brought it down from your Lord with truth) (16:102).
(And truly, this is a revelation from the Lord of all that exists, Which the trustworthy Ruh (Jibril) has brought down upon your heart that you may be (one) of the warners.) (26:192-194).
(A Book whereof the Ayat are explained in detail) means, its meanings are clear and its rulings are sound and wise.
(a Qur'an in Arabic) means, because it is a clear Arabic Qur'an, its meanings are precise and detailed and its words are clear and not confusing. This is like the Ayah:
((This is) a Book, the Ayat whereof are completed, and then explained in detail from One (Allah), Who is All-Wise Well-Acquainted) (11:1). meaning, it is miraculous in its wording and in its meanings.
(Falsehood cannot come to it from before it or behind it, (it is) sent down by the All-Wise, Worthy of all praise.) (41:42).
(for people who know.) means, this clear style will be readily understood by scholars who are thoroughly versed in knowledge.
(Giving glad tidings and warning,) means, sometimes it brings glad tidings to the believers, and sometimes it brings a warning to the disbelievers.
(but most of them turn away, so they hear not.) means, most of the Quraysh did not understand anything of it at all, despite the fact that it was so clear.
(And they say: "Our hearts are under coverings...") meaning, they are wrapped and screened,
(from that to which you invite us; and in our ears is deafness,) means, `we are deaf to the message you bring to us.'
(and between us and you is a screen,) `so nothing of what you say reaches us.'
(so work you (on your way); verily, we are working) means, go your way, and we will go our way, and we will not follow you.