What is preprocessor directive

Answer 1

I'd say "instructions which you give to the preprocessor", but for the sake of depth/detail:
https://www.tutorialspoint.com/csharp/csharp_preprocessor_directives.htm

Answer 2

in C++ language, preprocessors instructions ( those executed before compiling) are known by the identifier #. For exemple:
// includes the input/ output stream library
#include <iostream>
// defines a PI variable with value equal to 3.14
#define PI 3.14
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