What does `using std::swap` inside the body of a class method implementation mean?

I was trying to learn and adopt the copy-swap idiom following this thorough explanation on this question: the Copy-Swap Idiom.

But I found some code I had never seen: using std::swap; // allow ADL in this example

class dumb_array
{
public:
    // ...

    void swap(dumb_array& pOther) // nothrow
    {
        using std::swap; // allow ADL    /* <===== THE LINE I DONT UNDERSTAND */

        swap(mSize, pOther.mSize); // with the internal members swapped,
        swap(mArray, pOther.mArray); // *this and pOther are effectively swapped
    }
};

1. what does using std::swap; mean inside the body of a function implementation ?
2. what does ADL mean ?