static Value m_Val; 这句里的static是什么意思？怎么理解？

程序代码：
#include<iostream>

using namespace std;

class Value
{
public:
Value(int nVal) {m_nVal=nVal;printf("Call Value::Value(int nValue)\n");}
~Value() {printf("Call Value::~Value()\n");}
Value& operator=(int nVal)
{
m_nVal=nVal;
printf("Call Value::operator=\n");
return *this;
}
void Dump() {printf("Value::m_nVal=%d\n",m_nVal);}
protected:
int m_nVal;
};

class Base
{
public:
Base() {Init();}
virtual ~Base() {Release();}
virtual void Init() {printf("Call Base::Init()\n");}
virtual void Release() {printf("Call Base::Release()\n");}
virtual void Dump() {printf("Call Base::Dump()\n");}
};

class Derive:public Base
{
public:
Derive() {printf("Call Derive::Derive()\n");}
~Derive() {printf("Call Derive::~Derive()\n");}
virtual void Init() {m_Val=2;printf("Call Derive::Init()\n");}
virtual void Release() {printf("Call Derive::Release()\n");}
virtual void Dump() {m_Val.Dump();}
protected:
static Value m_Val;
};

Value Derive::m_Val=0;

void DestroyObj(Base * pOb)
{
pOb->Dump();
delete pOb;
}

void main()
{
Derive *pOb=new Derive;
DestroyObj(pOb);
}