Read the Crazyswarm paper to understand the different components of the system. A preprint is available here.
Learn more about the Crazyflie platform, e.g. by reading sections 1 and 2 of “Flying Multiple UAVs Using ROS”, Springer Book on Robot Operating System (ROS), 2017. A preprint is available here or by following the official getting started guide.
Learn about the Robot Operating System (ROS), e.g. by reading part 1 of “Programming Robots with ROS. A Practical Introduction to the Robot Operating System” by Morgan Quigley, Brian Gerkey, William D. Smart, O’Reilly, 2015.
If you use a motion capture for external localization, read the respective user manual. Proper tuning of the system is essential to achieve good tracking performance.