this的使用

#include"iostream"
using namespace std;
class myclass
{
public :
    myclass(int a,int b,int c);
    static void getsum();
private:
    int a,b,c;
    static int sum;

};
int myclass::sum=0;
myclass::myclass(int a,int b,int c)
{
this->a=a;
this->b=b;

this->c=c;
sum+=a+b+c;
}
void myclass ::getsum()
{
//cout<<a<<endl;//
cout<<"sum="<<sum<<endl;
}
void main()
{
myclass m(1,2,3);
m.getsum();
myclass n(4,5,6);
n.getsum();
myclass::getsum();


}
this->a=a;
this->b=b;

this->c=c;这三句是什么意思啊