وَإِنَّ رَبَّكَ لَهُوَ الْعَزِيزُ
...
And verily your Lord, He is truly the All-Mighty,
means, the One Who has power over all things, to subdue and control them,
...
الرَّحِيمُ
the Most Merciful.
means, towards His creation, for He does not hasten to punish the one who sins, but He gives him time to repent, and if he does not, then He seizes him with a mighty punishment.
Abu Al-Aliyah, Qatadah, Ar-Rabi` bin Anas and Ibn Ishaq said:
"He is Almighty in His punishment of those who went against His commands and worshipped others besides Him."
Sa`id bin Jubayr said:
"He is Most Merciful towards those who repent to Him and turn to Him."