What is Conditional Compilation? how to use #ifdef #else #endif block by macro name

L8Q3_Conditional_Compilation.c

#include<stdio.h>
# define BOY
int main()
{
    //Conditional Compilation
    # ifdef BOY
    printf("He is a boy.");

    # else
    printf("She is a girl.");

    # endif
    printf("\n Good guess!");

}