It literally means in Hebrew, when something is wound. When you wind around something, and it's wound up. So if you take, for example, a rope, and you only have a single strand, and you put a lot of pressure on it, you have to let go. But if you put a second strand in that's intertwined, wound, then a third strand, a threefold cord is very difficult to break. If you tie it in a granny [knot], it's apt to slip out but, if you tie it in a square knot, the stronger that you pull it, the tighter and stronger the knot becomes and, because it's threefold, it doesn't give way.