It is called the flagpole truck it is also known as the finial ball. It's purpose is to stop the flag from getting wrapped up around the pole and swings the flag around. Most people refer to the truck as "the ball on top of the flagpole". The pulley to raise and lower the flag is also attached to it. Early on in its history, when “truck” still meant “small wooden wheel,” it developed the specialized meaning of “a small wooden cap or ball at the head of a mast or flagpole,” usually with holes through which lines supporting sails (or flags) could be passed. The use of “truck” in this sense, which dates back to dates back to around 1626, undoubtedly came from the resemblance of the gizmo to a small wooden wheel.
Hope this helps! Any further details needed or questions feel free to message me
