In C programming language, the preprocessor is a program that processes the source code before compilation. It is a separate step in the compilation process and is not a part of the compiler. The preprocessor provides several facilities, including inclusion of header files, macro expansions, conditional compilation, and line control. Preprocessor directives are lines of code that begin with a hash symbol (#) and must be the first non-blank character. The C preprocessor provides several directives, including #define, #include, #undef, #ifdef, #ifndef, #if, #else, #elif, #endif, #error, and #pragma. The preprocessor can be used to define macros, which are brief abbreviations for longer constructs. The preprocessor reads the source code and replaces the macro templates with the appropriate macro expansion before the program is handed over to the compiler.