It means that because of all the evil people have committed, Allah Almighty has made their hearts become blind resulting in their loss of feelings. And as a result of the accumulation of their sins, a layer of stains has covered their hearts.
(But no! In fact, their hearts have been stained by all ˹the evil˺ they used to commit!( Al-Muttaffiffin 83: 14
أي أن من كثر الذنوب التي فعلها الناس، جعل الله تعالى قلوبهم تعمى (تصبح عمياءة حتى مات إحساسهم). و بسبب إصرارهم على فعل المعاصي و تراكم ذنوبهم صار هناك طبقة غشاوة على قلوبهم.
(كَلَّا ۖ بَلْ ۜ رَانَ عَلَىٰ قُلُوبِهِم مَّا كَانُوا يَكْسِبُونَ ﴿١٤ ) المطففين 83: 14