10.1. What is the C Prepressor
The C Preprocessor is a simple macro-expander that is run on
source code files before passing them to the compiler. Lines that
begin with the hash symbol '#' are directives to
the C preprocessor.
When you create a macro you assign a name to a C expression.
You can then use this name in your code just as you would have used
the expression. The preprocessor replaces all occurences of that name
with the expression.