boiling water will be poured down over their heads. With it will melt (or vanish away) what is within their bellies, as well as (their) skins.
meaning, when the boiling water --which is water that has been heated to the ultimate degree- is poured down over their heads.
Ibn Jarir recorded from Abu Hurayrah that the Prophet said;
إِنَّ الْحَمِيمَ لَيُصَبُّ عَلَى رُوُوسِهِمْ فَيَنْفُذُ الْجُمْجُمَةَ حَتَّى يَخْلُصَ إِلَى جَوْفِهِ فَيَسْلُتَ مَا فِي جَوْفِهِ حَتَّى يَبْلُغَ قَدَمَيْهِ وَهُوَ الصِّهْرُ ثُمَّ يُعَادُ كَمَا كَان
The boiling water will be poured over their heads and will penetrate their skulls until it reaches what is inside, and what is inside will melt until it reaches their feet. This is the melting, then he will be restored to the state he was before.
It was also recorded by At-Tirmidhi, who said it is Hasan Sahih.
This was also recorded by Ibn Abi Hatim, who then recorded that Abdullah bin As-Sariy said,
"The angel will come to him, carrying the vessel with a pair of tongs because of its heat. When he brings it near to his face, he will shy away from it. He will raise a hammer that he is carrying and will strike his head with it, and his brains will spill out, then he will pour the brains back into his head. This is what Allah says in the Ayah;
يُصْهَرُ بِهِ مَا فِي بُطُونِهِمْ وَالْجُلُودُ
With it will melt what is within their bellies, as well as (their) skins."
وَلَهُم مَّقَامِعُ مِنْ حَدِيدٍ