Some low light situations would include concerts, the night sky, sunsets, or anywhere/anytime when there is not enought light. Since there is not a great enough light source, you need to increase your camera's sensitivity to light. To do this you would increase the ISO. Then, you also would need to let more light in so you would make your Shutter speed slower so more light is let in. Also, to let more light in you open your aperature.
The only light source here is the light from the sparkler. With a slow shutter speed, the image is blurred and you can see the sparkler's light trail.
To learn more about shooting in low light click here.
