Preprocessor directives are special commands in programming languages like C and C++ that are processed before the actual compilation of the code begins. They typically start with a hash symbol (#) and are used to include files, define constants, or conditionally compile code. For example, the directive #include allows programmers to include external libraries or header files, making their code more modular and reusable. These directives help manage code complexity and improve efficiency. They do not produce any output themselves but influence how the compiler interprets the code. Common directives include #define for defining macros and #ifdef for conditional compilation, allowing developers to create flexible and adaptable programs.