Sometimes you just don't have (or want) a flash or enough ambient light. If you have a tripod or even just suitable surface to sit the camera down on (whatever it is) - as long as it's stable... You can make even the slowest lens work or add a splash of light where needed. Shoot at a low ISO to keep both noise down and exposure time up. You want to have the shutter open long enough to "paint" your subject(s) with the light.
Using something like a flashlight, the painting is merely passing the light over your subject. Usually you want to do this evenly and across the entire subject (but not necessarily). The source of light can have a great impact on your final image. If it's specular (or "hard") or diffuse, if it's a regular incandescent bulb (warm) or LED (cool), the brightness, etc. You'll have to experiment to find what works best.
Consider the following two examples. The first had the camera sitting on a drinking glass. A flashlight was used to add just a touch of light on the cat. The second shot was a bit more extreme. Tripod mounted, a flashlight was used to increase light on the instrument, but done in such a way as to appear to be coming from the only light source in the room - the light just to the right. In both cases, there was neither enough ambient light nor enough light on the subject.
Not the best examples, nor a typical situation you might see light painting... But I've recently become fond of this technique, so I'll be posting more in this thread.
M8, ISO 640, Voigtländer 35/1.2 Aspherical Nokton, 4s @ f/22:
M9, ISO 160, Noctilux-M 50mm f/0,95 ASPH, 16s @ f/16: