The thread holder is a nifty article to use and a very handy thing to have.
The thread holder is there to hold the threads in place after threading them through the heddles. It is a small gadget which clips on any shaft frame. After hooking the thread through the heddle, instead of just letting the thread hang, clip it over the holder (see pic). The thread will be hold there neat and secure until your bundle of thread is finished. The thread holder can be moved along the shaft frame and afterwards transferred to the next loom.
I certainly couldn't thread without my thread holder any more.
The thread holder is made out of Australian stainless steel and hand made by BB Yarn Supply.