cpp

Cpp is the GNU C-Compatible Compiler Preprocessor.
Cpp is a macro processor which is used automatically
by the C compiler to transform your program before actual
compilation. It is called a macro processor because it allows
you to define macros, abbreviations for longer
constructs.

The C preprocessor provides four separate functionalities: the
inclusion of header files (files of declarations that can be
substituted into your program); macro expansion (you can define macros,
and the C preprocessor will replace the macros with their definitions
throughout the program); conditional compilation (using special
preprocessing directives, you can include or exclude parts of the
program according to various conditions); and line control (if you use
a program to combine or rearrange source files into an intermediate
file which is then compiled, you can use line control to inform the
compiler about where each source line originated).

You should install this package if you are a C programmer and you use
macros.