From the official Website (ROS.org | Powering the world's robots):
"The Robot Operating System (ROS) is a set of software libraries and tools that help you build robot applications"
I don't think there is a ROS model for Tello, I might be wrong though. Only thing I found is the DJI-sdk package (dji_sdk - ROS Wiki)
but it doesn't support Tello from what I understand