In C programming, preprocessors are directives or commands that are used to give instructions to the compiler before the actual compilation process begins. Preprocessors provide a way to perform tasks such as including header files, defining constants, and performing conditional compilation.

The preprocessor directives begin with a hash (#) symbol, which is followed by the directive name and any necessary arguments. Some common preprocessor directives include:

1. #include - used to include header files in the source code
2. #define - used to define constants or macros
3. #ifdef and #ifndef - used for conditional compilation
4. #pragma - used to provide additional instructions to the compiler

Preprocessor directives are processed before the actual compilation process begins. The preprocessor replaces the directive with the appropriate code, which is then compiled by the compiler. This allows programmers to write more efficient and maintainable code.