Oh, swingletree! It's like a tool that helps horses pull stuff. When a horse is pulling a wagon or plow or something, the swingletree connects to the harness that the horse is wearing. Then the other end of the swingletree connects to the thing that the horse is pulling.
The swingletree helps make it easier for the horse to pull the thing because it allows the horse to move around a little bit without pulling the thing all wonky. It keeps everything straight and balanced.
So basically, the swingletree is like a helper for the horse when it's doing hard work like pulling a heavy load. It helps the horse do the job better and more comfortably.