A Cable Shutter Release, henceforth refered to as a CSR is a small, electronic gadget, generally consisting of plastic or metal. It attaches to your camera via a cable.

It functions like a second shutter button, so if you need to keep your camera still, you can position it on a tripod and use a CSR to avoid camera shake.

It is often used with BULB mode ( shutter remains open for as long as you depress the shutter button), so if you are photographing fireworks you can set your camera up on a tripod, attached the CSR and use bulb mode for a longer exposure. Just press the CSRs button and a picture will be taken. Most CSRs also feature the ability to lock, so if you are doing star trails, you can leave the camera shutter open without pressing the button all the time, and you can grab a coffee and relax!