Early in the compiling of a source, a preprocessor takes these
# directives.
#include basically puts all of stdio.h into the top of your code. the .h identifies it as a "header".
In C, before you use a function, the compiler must first know how it is suppose to be called.
For example, how many arguments and of what type? All this is in stdio.h, for the Standard I/O functions (things that print to the screen and such).
Usually, in C++ it's
#include <iostream>, but we won't fault you for using C functions