The C preprocessor is a tool which filters your source code before it is compiled. The preprocessor allows constants to be named using the #define notation. The preprocessor provides several other facilities which will be described here. It is particularly useful for selecting machine dependent pieces of code for different computer types, allowing a single program to be compiled and run on several different computers.
The C preprocessor isn't restricted to use with C programs, and programmers who use other languages may also find it useful, however it is tuned to recognise features of the C language like comments and strings, so its use may be restricted in other circu mstances.
The preprocessor is called cpp, however it is called automatically by the compiler so you will not need to call it while programming in C.