A turnbuckle is used to hold and tighten a steel wire rope so that the wire can only move in one direction. They can also be used to fasten a mast to so-called stays to stabilize the mast, and thus the boat.
You unscrew the two "forks" from each other and tighten them with a nut. Then you mount it in the desired location and tighten the turnbuckle by inserting a thin screwdriver or similar through the small hole in the middle and turning until the wire is fully tensioned.
