what is 'using' keyword in c++???

Answer 1

There are namespaces in programs, think of std namespace which is declared in standard library.
If you'd like to use a variable or object from a declared namespace you have to use scope operator.
For example

std::cout << "Hello!;

With using you gives that you use a given namespace so you have not need a scope operator.

For example:

using namespace std;
cout << "Hello!";