Chapter 32: Verse 21
"The greater chastisement" here refers to the chastisement to which the unbelievers and evil-doers will be subjected in the World-to-Come. The verse, however, also speaks of "some chastisement" in this world, which refers to things like serious diseases, death of kith and kin, tragic accidents, and losses and failures of various types which people face in the course of their worldly lives. There are also storms, earthquakes, floods, epidemics, famines, riots, warfares and other calamities which befall a people collectively, sometimes affecting the lives of millions.
These calamities afflict people in order that they may take heed before the greater chastisement overtakes them. They should give up those patterns of thought and action that lead them to the immense chastisement of the Hereafter. In other words, God has not made man's life in the world to be spent without care and concern. Man has not been granted an altogether smooth sail in this life. Calamities visit him so that he may purge his mind of the delusion that there is no power above him capable of causing harm.
They are thus made to realise that their destiny is controlled by someone else other than themselves. This real power and authority rests with God, not with man. Whenever any calamity from God strikes man it becomes evident that he can neither avert it himself nor by invoking any jinn, spirit, or god. Far from being simply natural disasters, these calamities serve as warnings from God to dispel man's misperceptions and prompt him to recognise the reality of things. By deriving lessons from these, man can mend his ways during his existence by embracing right beliefs and reforming his conduct. This alone will save him from the greater chastisement of the Hereafter.
"Towards Understanding the Quran" - Sayyid Abul Ala Mawdudi, Vol. 8, p. 173