A preprocessor is a tool used in programming that processes source code before it is compiled. It typically handles tasks like macro substitution, file inclusion, and conditional compilation. This allows developers to write more flexible and maintainable code by enabling features like code reuse and configuration management. In languages like C and C++, the preprocessor is an essential part of the compilation process. It reads directives, which are special commands that begin with a hash symbol (#), such as `#include` for including files or `#define` for defining constants. This preprocessing step helps streamline the code before it reaches the compiler.