why we use using namespace std;

Answer 1

in the most simple form
using namespace std;

means just resolving scope of the objects in the standard library of C++ like cout or cin

more simply it's just saying the compiler that "I'm using the objects of the standard library"

if you won't use that line the when doing the output you'll write,
std::cout<<"Hello world";
instead of
cout<<"Hello world";

you'll learn about it in the latter parts of the Tutorial

Answer 2

to declare namespace
if you won't write it, you need to write functions of iostream library like std::cout, std::cin etc.