Tafsir of Al-Kahf 18:2

قيما

(He has made it) straight, meaning straightforward,

لينذر بٲسا شديدا من لدنه

to give warning of a severe punishment from Him, meaning, to those who oppose His Prophet and disbelieve in His Book, He issues a warning of severe punishment hastened in this world and postponed to the world Hereafter

من لدنه

(from Him). means, from Allah. For none can punish as He punishes and none is stronger or more reliable than Him.

ويبشر المومنين

and to give good news to the believers, means, those who believe in this Qur'an and confirm their faith by righteous actions.

الذين يعملون الصالحات

who do righteous deeds,

أن لهم أجرا حسنا

that they shall have a fair reward. means, a beautiful reward from Allah