The heap is an area of memory that is managed by the operating system.
When you put data on the heap, you request a certain amount of space. The memory allocator finds an empty spot in the heap that is big enough, marks it as being in use, and returns a pointer, which is the address of that location. This process is called allocating on the heap.
Memory on the heap is allocated, deallocated, and resized regularly during program execution, and this can lead to a problem called fragmentation. Fragmentation occurs when memory objects are allocated with small spaces in between that are too small to hold additional memory objects. The result is a percentage of the heap space that is not usable for further memory allocations.