= delete - what does this do?

I was looking at the std thread class for c++:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2497.html#thread.threads

I was wondering what the "= delete" does when declaring a constructor?

--> thread(const thread&) = delete;

Thanks

{ See the paper titled "Defaulted and Deleted Functions" at
   <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2326.html>.
   -mod/sk }

--------

class thread
{
public:
     // types:
     class id;
     typedef implementation-defined native_handle_type; // See
[thread.native]

     // construct/copy/destroy:
     thread();
     template <class F> explicit thread(F f);
     template <class F, class ...Args> thread(F&& f, Args&&... args);
     ~thread();
     thread(const thread&) = delete;
     thread(thread&&);
     thread& operator=(const thread&) = delete;
     thread& operator=(thread&&);

     // members:
     void swap(thread&&);
     bool joinable() const;
     void join();
     void detach();
     id get_id() const;
     native_handle_type native_handle(); // See [thread.native]

     // static members:
     static unsigned hardware_concurrency();
};

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