> but i really don't understand what copying a unique_ptr implicitly would mean.
The author is imagining box would be like unique_ptr but with an implicit copy constructor that copies the heap data:
unique_ptr(unique_ptr<T>& o) : unique _ptr(*o) {}
Then if you have this code:
function f(unique_ptr<int> ptr)
{ cout << *ptr; }
f(some_uniq); // implicitly copies the uniqueptr and its data
