This is a Dynalite Sync Cord. One end is a 1/4" "Phono" style plug that fits into any compatible power pack or monolight, the other a PC (Post Connector) plug that fits into your camera. It enables firing of your flash from your camera from up to 16' (5 m) away.
In the Box
Dynalite Sync Cord, 1/4" Diamater Phono to PC - 16' (5m)