Here Allah tells us about His servant and Messenger Musa, peace be upon him, and how He sent him to Fir`awn and his chiefs. That is, his governors, ministers, leaders and followers among the Egyptians and the Children of Israel. Allah sent him to call them to worship Allah alone, with no partner or associate, and to forbid them from worshipping anything other than Him. He sent him with mighty signs, such as his hand and his staff, other signs such as the flood, locusts, Qummal, frogs and blood, and the loss of their crops and lives. Yet despite all that they remained arrogant and stubbornly refused to follow him; they rejected his message and made fun of it, and laughed at the one who had brought it.

وَمَا نُرِيِهِم مِّنْ ءَايَةٍ إِلاَّ هِىَ أَكْبَرُ مِنْ أُخْتِهَا

(And not an Ayah We showed them but it was greater than its fellow preceding it,) but despite that they would not give up their sin, misguidance, ignorance and confusion. Every time one of these signs came to them, they would go and implore Musa, saying,

يأَيُّهَ السَّاحِرُ

("O you sorcerer!...") meaning, expert or knowledgeable one -- this was the view of Ibn Jarir. The scholars of that time were the sorcerers or magicians, and magic was not regarded as something reprehensible by them at that time, so this was not a slight on their part. They were in a state of need, so it would not have been appropriate for them to insult him. This was a way of honoring him, as they thought. On each occasion, they promised Musa that if the torment was lifted from them, they would believe in him and let the Children of Israel go with him, but on each occasion they went back on their word. This is like the Ayat:

فَأَرْسَلْنَا عَلَيْهِمُ الطُّوفَانَ وَالْجَرَادَ وَالْقُمَّلَ وَالضَّفَادِعَ وَالدَّمَ ءَايَـتٍ مّفَصَّلاَتٍ فَاسْتَكْبَرُواْ وَكَانُواْ قَوْماً مُّجْرِمِينَ - وَلَمَّا وَقَعَ عَلَيْهِمُ الرِّجْزُ قَالُواْ يَمُوسَى ادْعُ لَنَا رَبَّكَ بِمَا عَهِدَ عِندَكَ لَئِن كَشَفْتَ عَنَّا الرِّجْزَ لَنُؤْمِنَنَّ لَكَ وَلَنُرْسِلَنَّ مَعَكَ بَنِى إِسْرَءِيلَ - فَلَمَّا كَشَفْنَا عَنْهُمُ الرِّجْزَ إِلَى أَجَلٍ هُم بَـلِغُوهُ إِذَا هُمْ يَنكُثُونَ

(So We sent on them: the flood, the locusts, the Qummal, the frogs, and the blood; manifest signs, yet they remained arrogant, and they were of those people who were criminals. And when the punishment struck them, they said: "O Musa! Invoke your Lord for us because of His promise to you. If you remove the punishment from us, we indeed shall believe in you, and we shall let the Children of Israel go with you." But when We removed the punishment from them to a fixed term, which they had to reach, behold! they broke their word!) (7:133-135)