In an effort to explain how some light plus a little more light becomes even more light, let me explain how this might matter in the first place. Let’s say you’re photographing a portrait in a studio setting, so you set up a light to illuminate the subject. (It could be a strobe, a hot light, an LED, in a softbox, with a snoot—as long as it’s a light, it applies.) This light, which provides the primary illumination for the subject is the key light, or main light. In a picture made outdoors, the sun is almost always the key light.
So now you’ve got a light set up and aimed at your subject. You could take a meter reading of the amount of light illuminating your subject, and let’s say your main light reads 1/125th at f/8 and ISO 100. Set your camera this way and you’ll have a nice, normal, well-exposed photo.
Now let’s say you add to this scenario a fill light. This is a secondary light that’s reaching the subject with less intensity than the main light and is intended to illuminate shadows—maybe a little bit or maybe a lot, depending on the look you’re going for. Without fill, shadows are deep and dark and dramatic, and that’s not always the look you want. Especially for, say, a nice, friendly portrait. So you add a fill light, placed as close to the camera as possible so as not to create a new set of competing shadows from a secondary main light—a phenomenon known as “cross lighting.” Adding a fill light from the camera’s position is ideal because, after all, we don’t care about shadows the camera can’t see. A light from the camera will fill in all the shadows the camera can see.