The line #include <iostream> tells the compiler to include code from the iostream library. This library contains cout, which prints output. Without this line, the compiler does not know what cout means.
Try deleting #include <iostream> and clicking run. You get an error saying cout is not declared. The compiler cannot use code it has never seen. Libraries save you from writing everything yourself.
Instead of writing hundreds of lines to print text, you include iostream and use cout.