understanding calling of variables

calling variables ...

‪#include<iostream.h>
#include<conio.h>
int a=2;   // global variable

void main()
{
int a=5;  //  local variable
cout<<"value is:"<<a<<endl;
cout<<"value of global variable is:"<<::a;
getch();
}
Output

value is:5
value of global variable is:2