A .hh file is a header file used primarily in C++ programming. Header files contain declarations of functions, classes, variables, and other programming constructs. They serve as an interface between different parts of a program, allowing code in one file to access and use elements defined in another. The compiler uses header files to understand the structure and relationships within the code. Including a header file in a source file essentially imports the declarations it contains, enabling the use of those declared elements. Header files are crucial for code organization, reusability, and maintainability in C++ projects. They promote modularity by separating interface definitions from implementation details. When a program is compiled, the compiler uses the information in the header files to ensure that all function calls and variable references are valid. Changes to header files often require recompilation of dependent source files.