A build tree means a directory tree of files generated by CMake, compilers, linkers etc. See generate a project build system.
For example:
MyBuildTree └── Debug ├── CMakeFiles ├── CMakeCache.txt ├── build.ninja ├── cmake_install.cmake └── rules.ninja
The cmake variable CMAKE_BINARY_DIR represents the full path to the root folder of the build tree.
An out-of-source build tree is recommended.