CPM stands for CMake Package Manager which is a convenience wrapper over CMake's FetchContent module. You can download the CPM.cmake from the project releases page.
With CPM enabled, you can bring this library in using the following command:
CPMAddPackage("gh:baderouaich/BitmapPlusPlus#master")
And link it to your target using:
target_link_libraries(${TARGET} LINK_PRIVATE bpp::BitmapPlusPlus)
Note that your target needs to compile with C++17 or newer. After that, you can simply include the library in your code:
#include <BitmapPlusPlus.hpp>