The document discusses heap sort, which is a sorting algorithm that uses a heap data structure. It works in two phases: first, it transforms the input array into a max heap using the insert heap procedure; second, it repeatedly extracts the maximum element from the heap and places it at the end of the sorted array, reheapifying the remaining elements. The key steps are building the heap, processing the heap by removing the root element and allowing the heap to reorder, and doing this repeatedly until the array is fully sorted.