[1] A heap is a particular way of ordering the elements in a range of Random Access Iterators [f, l). The reason heaps are useful (especially for sorting, or as priority queues) is that they satisfy two important properties. First, *f is the largest element in the heap. Second, it is possible to add an element to a heap (using push_heap), or to remove *f, in logarithmic time. Internally, a heap is a tree represented as a sequential range. The tree is constructed so that that each node is less than or equal to its parent node.

[2] Pop_heap removes the largest element from a heap, and shrinks the heap. This means that if you call keep calling pop_heap until only a single element is left in the heap, you will end up with a sorted range where the heap used to be. This, in fact, is exactly how sort_heap is implemented.