This is a device for tightening guy wires. Since one side has left-handed threads, a turnbuckle will tighten the guy wire without twisting it. For the type pictured below, you must tape over the turnbuckle after adjusting it or vibration will cause it to loosen with time.
http://www.hdtvprimer.com/ANTENNAS/turnbuckle.jpg
http://www.hdtvprimer.com/ANTENNAS/turnbuckle.jpg