Catchlights are the bright, specular highlights found in a subject's eyes that are reflections of the light source or sources. They will appear the same shape of the light modifier used. When a portrait is lit with available light, the sky, a window, or other available light source can be seen reflected in the eye, however such a soft or even dull light source may not create that "sparkle" in the subject's eye, like an added light source can.
The importance of having a catchlight in your portraits
One mistake new portrait photographers make is to find soft, gentle light, then assume they can photograph subjects naturally, facing in any direction, simply because they're not in harsh light.
Unfortunately, even on an overcast day with soft light, your portraits may turn out gloomy and dull if the subjects' eyes do not have a catchlight! A catchlight can be created simply by having your subjects face a naturally bright light source, or by adding a reflector or flash to the image.
Therefore, it is almost always a good idea to ensure that you have at least one light source, be it natural or artificial, that is strong enough to create a "sparkle" in your subjects' eyes.
How to create a catchlight
To create a catchlight, simply position a strong light source at an angle between you and your subject that allows the light source to be reflected in the subjects' eyes. The most common mistake is placing the light source too high, in which case the subjects' eyelids and eyelashes may obscure most or all of the catchlight.
The easiest way to ensure you have a catchlight, on the other hand, is to place the light source slightly *beneath* your camera's position, or just to the left or right of the camera, depending on your desired lighting effect.