Here is a quick guide to what backlight is and how it works.
Backlight can be a difficult concept as it is used in a variety of different situations for different reasons. One of the most common locations to see a backlight is in a three point lighting scheme for an interview on film or television. Here it is used to highlight the back of the head to separate it from the background, add definition to the subject, and give then a certain ambiance that help on screen. Backlight in general is used in many more situations than that and has a few principles behind it.
The Digital Video Backlight Set Up
Backlight is essentially what it sound like: light that comes from behind the subject. It is primarily light that is directed toward the camera's position, but blocked by the subject. This creates a glow that is variable for the subject. Backlight can be created artificially with a studio or light kit, and can also be harnessed in a natural environment by positioning the subject in relation to natural light sources. You are likely going to want to use a reflected light meter in these situations.
You are going to have to go with your instincts usually when trying to position the backlight effectively. For example, a natural light source like a window can be useful if it is not too large. If it is too large then you will end up seeing that the brightness of an open window source can blow out the image if it stays in the frame.
When outside you will usually want the subject to block the sun directly. This way it will be an effective backlight while also keeping the sun out of the subjects eyes. If it is really bright then you may want to avoid this and instead go for using a reflector both on the spot and the backlight.
Studio set ups are always the best choice, and on a set up on a subject the backlight is usually the difference between amateur and professional images. Many people will tell you that you do not want too strong of a backlight, but it is nice to really be able to see it. Try to rotate the backlight so it is not directly on the back of the subjects head. You can just rotate this additionally until you get the level of intensity you want.
It is fine to get the backlight out of the frame if you need to, but do not bring it too far off otherwise the light source will appear as though it came from the side instead of the back.