why do we return 0 in c++ code example

Example: return 0; c++

// This example gives a reason why you should use using namespace std
#include <iostream>
using namespace std;
cout << "Hello "<<endl; /* this one is shorter*/
// without the using namespace std you will need to use this
std::cout <<"Hello"<<std::cout<<endl /* this one is longer 
and it requires shorter amount of time */