The file "CMakeLists.txt" is a configuration file used by the CMake build system. It contains instructions that define how a software project should be built, including specifying source files, libraries, and compiler options. This file allows developers to manage complex builds across different platforms and compilers easily. In a typical CMake project, multiple "CMakeLists.txt" files can exist, with a main file in the root directory and others in subdirectories. Each file can include commands to organize the project structure, set dependencies, and control the build process, making it a crucial component for software development.