This is a 16.5' straight extension cord for an existing sync cord. A sync cord provides the necessary connection between a power pack or monolight, and a camera, or triggering device that has a female PC (post connection) terminal.
Straight cords are lighter, and usually more economical than coiled cords; coiled cords stretch when necessary, and are more heavy duty than straight cords.
Premium sync cords have gold-plated ends, for longer lasting sure connections.