Heddles are weaving loom needles and a weavers threading nightmare. Multiply threading one needle by...let's be conservative and say 50....times 4 Harnesses. Harnesses being the frames that hold top and bottom rails that the needles slide on. Yes, they are moveable and threaded one at a time through the center of each vertical needle. A threading tool with a hook on the end is used to pull threads through the eye.
Briefly, heddles are essential pieces of a weaving loom whether they be on a Harness Rail or a Ridgid Heddle frame. I have both and I have to say the Ridgid Heddle Loom is much less of a nightmare to setup and thread. In fact in comparison it is a Dream.
However, it is limited somewhat to a plain weave or Tabby Weave.