The Skein of Thread was an enchanted thread that belonged to Ariadne. It was attached to a necklace and would never tangle or break.
It was originally enchanted by the Witches of Caucus. Ariadne gave the skein to Jason in order for him to find his way into the Labyrinth and out again. Jason tied one end to the cave entrance and was able to travel through the caves without getting lost. He returned the thread back to Ariadne and he was grateful for it. (The Earth Bull)