A ".dll" file, short for Dynamic Link Library, is a type of file used in the Microsoft Windows operating system. It contains code and data that multiple programs can use simultaneously, allowing for efficient memory usage and modular programming. This means that developers can create applications that share common functions without needing to duplicate code in each program. When a program runs, it can call upon the functions stored in a .dll file, which helps streamline the software's performance. If a .dll file is missing or corrupted, the associated program may not work correctly, leading to error messages or crashes.