Header files are files in programming, typically with a `.h` extension, that contain declarations for functions, variables, and other identifiers. They allow programmers to share code between different source files, promoting code reusability and organization. By including a header file in a source file, developers can access the functions and variables defined in that header without needing to rewrite the code. In languages like C and C++, header files are crucial for managing large projects. They often contain function prototypes, type definitions, and macros. Using header files helps to separate the interface of a module from its implementation, making code easier to maintain and understand.