On a floor loom every warp thread goes through a heddle. On old looms it was probably made of string with knots at the right places to hold the warp thread at the correct level. These days most looms have metal heddles with a slot or eye in the middle to hold the warp thread.
The flat heddle on the left has sharper edges and can be rough on a warp thread that is large or hairy but it will sit very close to it's neighbors and allow a very tight sett. The inserted eye heddle on the right is much kinder to warp threads but won't take as large a thread or pack so closely together if you want a very tight sett.
I'm usually very careful when I thread the heddles. Warping is my least favorite part of the weaving process so I would be happier if it took less time but you can't rush it either. If you get one warp thread in the wrong heddle and don't notice it until you have the rest of your warp finished it is a huge amount of work to tear it all out and do it again. So I'm careful. I guess I'm better at threading the heddles than I realized because for the first time I needed to make a repair heddle.
Regardless of what it sounds like, a repair heddle is not to fix a broken heddle. It's to fix a mistake in threading the heddle. In this case I had one warp thread that should have been threaded into a heddle on the 5th harness and I threaded it into a heddle on the 6th harness. Oops. I figured it out when I started weaving and realized there was a mistake in the pattern. At that point I had three choices. I could keep weaving and live with the mistake in the pattern. I could tear out and re-threaded about 200 warps threads. Or I could make a new heddle and fix the mistake by re-threading the one warp thread that was wrong.
The repair heddle is made with a safety pin at each end so it can be attached to the harness. The center of the new heddle is made of string or in this case cottolin weaving yarn that is strong and not at all stretchy.
Using t-pins to tie against, the center eye is created with a couple of strong knots. In the end you get a new heddle you can slip onto the harness frame and thread as needed.
Once the warp thread has been removed from the heddle in the wrong location it is threaded through the new heddle. Voila! The warp is exactly where it would have been if I had been paying more attention in the beginning.
By the way, I had to look up how to make my repair heddle. I knew I needed to make one but not how to do it. It took me just a few minutes and was totally satisfyingly wonderful.