The challenge with outdoor portraits is how to ensure that your subject’s face is well lit. Often you’ll get harsh shadows in areas which don’t have direct light on them. Such shadows create dead patches in your image and can actually cause certain facial features to be accentuated in non flattering ways.
Commonly it is the undersides of a face that is the problem area in outdoor portraits (under the chin, nose etc).
Using a reflector helps with this by reflecting available light into those areas of your subjects face that would be in shadow without it. Quite often the best place for one is below your subjects face reflecting light back up into those dark under-spots.
The most obvious place to try out a reflector is outside but don’t write them off for indoor shooting. I find that they are particularly handy in those shots you take indoors next to windows where there is some natural light, but not quite enough to shoot without a flash. Introduce reflector into these situations and you will often find that a flash is not needed at full strength (if at all).
Reflectors generally come in two colours, silver and gold. Each gives off a different light, silver ones giving a bright and whiter reflection and gold giving a warmer and more subtle light.
To use a reflector you’ll usually want to get it pretty close to your subject – without getting it in the shot. Place your subject so that they are not looking directly into the sun (I like back or side light) and then position the reflector so that it’s glow bounces back up to light up your subject’s face. You’re ideally looking for a nice even light with no shadows so keep positioning the reflector until you achieve this (an assistant can be handy with this – although if your shot is more tightly cropped you might even be able to get the reflector positioned in your subjects lap).