Why is this defined in this header ?

Why is NODE_H defined in the code below ?

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#ifndef NODE_H
#define NODE_H
#include <iostream>

using namespace std;

template<class T>
class Node
{
    public:
        Node();
        Node(const T& item, Node<T>* ptrnext=NULL);
        T data;
        Node<T>* NextNode();
        void InsertAfter(Node<T>* p);
        Node<T>* DeleteAfter();
        Node<T>* GetNode(const T& item, Node<T>* nextptr= NULL);
    private:

        Node<T>* next;
};

#endif 
The name .h file...
Thanks.
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