Commentary
After having shown the possibility and proof of 'Akhirah or Hereafter, Allah Ta’ ala has described some events relating to Resurrection in these verses.
Given in the first verse (19) is the mode in which the dead will rise again. It was said: فَإِنَّمَا هِيَ زَجْرَةٌ وَاحِدَةٌ, it will be only a single [ castigating ] Call and all of a sudden they will begin to see - 37:19). The word: زَجْرَةٌ (Zajrah) is a noun derived from زَجر (zajr), and it is used to carry many meanings in the Arabic language. Out of these, one meaning is: 'To utter such sounds which, when heard by cattle, would motivate them to rise and get moving.' Here, it means the second Sur (Horn or Trumpet) that will be blown by the angel, Sayyidna Israfilll. Hence, it has been expressed as 'zajrah' for the reason that the way some sounds are produced to prompt cattle to rise and get moving, very similarly, this Sur will be blown to make the dead rise.
Though Allah Ta’ ala is, in His infinite power, inherently capable of making the dead rise again without having the صُور Sur blown, yet this صُور Sur will be blown in order to highlight the awe of the Resurrection (Tafsir Kabir). Its effect on the disbelievers has been identified as: فَإِذَا هُمْ يَنظُرُونَ all of a sudden they will begin to see - 37:19), that is, the way they could see in the mortal world, in the same way, they would be able to see there.
Some other commentators have explained it by saying that, overtaken by wonder, they will start looking at each other. (Qurtubi)