Cord Magic is literally what is sounds like... Using cords, thread, rope, yarn, string, twine, vines, hair (I don't know your life) to create magic.
Before there were manufactured closures for garments, there was yarn and string and ribbon to hold our clothing in place. Before there were chains and locks, there were handmade ropes and hand tied knots to hold a boat at the dock.
And, as you probably know, there has always been Magic.
While some of the examples I've read about could certainly be passed off as superstition (like tieing certain knots in your sails to ward off wiley mermaids that might lure you to a watery grave) the process and intention is what matters.
Cord Magic is about binding your intention into reality.