What does deleting a pointer mean?

Yes, delete is used to deallocate memory and call the destructor for the object involved.

It's common pratice to set pointer to NULL after deleting it to avoid having invalid pointers around:

Object *o = new Object();

// use object
delete o; // call o->~Object(), then releases memory
o = NULL;

When new and delete are used with standard C types in C++ source they behave like malloc and free.

Deleting a pointer (or deleting what it points to, alternatively) means

delete p;
delete[] p; // for arrays

p was allocated prior to that statement like

p = new type;

It may also refer to using other ways of dynamic memory management, like free

free(p);

which was previously allocated using malloc or calloc

p = malloc(size);

The latter is more often referred to as "freeing", while the former is more often called "deleting". delete is used for classes with a destructor since delete will call the destructor in addition to freeing the memory. free (and malloc, calloc etc) is used for basic types, but in C++ new and delete can be used for them likewise, so there isn't much reason to use malloc in C++, except for compatibility reasons.

You can't "delete" a pointer variable, only set their value to NULL (or 0).

You can't "delete" a pointer variable

Sure you can ;-)

int** p = new int*(new int(42));
delete *p;
delete p; // <--- deletes a pointer

But seriously, delete should really be called delete_what_the_following_pointer_points_to.