(My Lord will make another people succeed you, ) This refers to a group of people who will worship Allah alone, without associating anything with Him. This also implies that the polytheists do not bother Allah and they do not harm Him in the least with their disbelief. To the contrary, their disbelief merely harms their own selves.
إِنَّ رَبِّى عَلَى كُلِّ شَىْءٍ حَفِيظٌ
(Surely, my Lord is Guardian over all things.) This means that Allah is a Witness and Guardian over the statements of His servants and their actions. He will give them due recompense for their actions. If they do good deeds, He will reward them with good. If they do evil, He will punish them with evil.
The Destruction of the People of `Ad and the Salvation of Those among Them Who believed
وَلَمَّا جَآءَ أَمْرُنَا
(And when Our commandment came,) This is referring to the barren wind with which Allah destroyed them, to the very last of them. The mercy and kindness of Allah, the Exalted saved Hud and his followers from this terrible punishment.
فوَتِلْكَ عَادٌ جَحَدُواْ بِآيَـتِ رَبِّهِمْ
g(Such were `Ad (people). They rejected the Ayat of their Lord) This means they disbelieved in the proofs and revelations (of Allah) and they disobeyed the Messengers of Allah. This is due to the fact that whoever disbelieves in a Prophet, then verily, he has disbelieved in all of the Prophets, peace be upon them. There is no difference between any one of them, in the sense that it is necessary to believe in all of them. Therefore, `Ad disbelieved in Hud and their disbelief was considered disbelief in all of the Messengers.
وَاتَّبَعُواْ أَمْرَ كُلِّ جَبَّارٍ عَنِيدٍ
(and followed the command of every proud, obstinate.) This means that they abandoned following their rightly guided Messenger and they followed the command of every proud, obstinate person. Thus, they were followed in this life by a curse from Allah and His believing servants whenever they are mentioned. On the Day of Resurrection a call will be made against them in front of witnesses.