CMakeLists.txt is the build configuration file. It describes what to build, where sources are, dependencies, and build options. Written in CMake's scripting language (imperative, case-insensitive commands).

Minimal example:

cmake_minimum_required(VERSION 3.20)
project(MyApp)
 
add_executable(myapp main.cpp)

This declares a project named MyApp (creates a binary myapp from main.cpp). Run cmake . to generate build files in the current directory (or cmake -B build for out-of-source).

Common commands:

cmake_minimum_required(VERSION X.Y)  # minimum CMake version
project(name LANGUAGES C CXX)        # project name, languages used
add_executable(name source.cpp)      # executable target
add_library(name source.cpp)         # library target
target_link_libraries(exe lib)       # link target to library
find_package(PackageName)            # find external package
include_directories(/path/to/headers) # add include path
set(VARIABLE value)                  # set variable

Commands are functions: command(arg1 arg2 ...). Lists use semicolons or spaces. Variables are ${VAR}. Comments start with #.

Structure:

1. cmake_minimum_required (must be first)
2. project() declaration
3. find_package() calls for dependencies
4. add_executable() or add_library() for targets
5. target_link_libraries() and other target configuration
6. install() rules (optional)

CMake scans source files for dependencies. It's declarative about what you want to build; the generator handles how to actually compile it for your platform.