Difine #include <<iostream>>

Question

quick responce

Answer 1

The built in C++ library routines are kept in the standard namespace. That includes stuff like cout, cin, string, vector, map, etc. Because these tools are used so commonly, it's popular to add "using namespace std" at the top of your source code so that you won't have to type the std:: prefix constantly

Answer 2

Firstly it is called define. I'm sorry sometimes I'm a GrammarNazi (while living in Germany...coincidence? 🤔)
Well back to the topic
With #include you tell your c/c++ program that it should load some additional files. in this particular case if you don't include iostream (in C stdio.h) you won't able to print anything on your console or getting input from your keyboard or file etc. In order to work properly with comandline outputs and inputs it is essential to add this line of code.
There are lots of different files which can be included with different functions for different purposes.
You have also the possibility to include header files by #include