What is Cable Release?
A cable release is a device - generally a cord with an input on one end and a small remote on the other - that attaches to your camera and triggers the shutter
to fire, as an alternative to pressing the shutter release button on your camera.
A Little More Detail:
Cable release is ideal if you’re shooting with a tripod, as it reduces the risk of ‘camera shake’ that can happen when the shutter button is depressed.