What are proxy objects? - c++ interview question with solution

What are proxy objects? 
Objects that stand for other objects are called proxy objects or surrogates. 
template 
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index)const;
};

Array1D operator[] (int index);
const Array1D operator[] (int index) const;
};

The following then becomes legal:

Array2Ddata(10,20);
cout<