Tafsir of Fussilat 41:3

كتاب فصلت آياته

A Book whereof the Ayat are explained in detail, means, its meanings are clear and its rulings are sound and wise.

قرآنا عربيا

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:

كتاب أحكمت ءايته ثم فصلت من لدن حكيم خبير

(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

ﻻ يٲتيه البطل من بين يديه وﻻ من خلفه تنزيل من حكيم حميد

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)

لقوم يعلمون

for people who know. means, this clear style will be readily understood by scholars who are thoroughly versed in knowledge