Light from any source – a window, a continuously burning lamp, a flash – foltows the same general rule: The light falls off (gets dimmer) the farther the light source is from an object. But light from a flash comes and goes so fast that you can’t see the effect of the flash on a scene at the time you are taking the picture. Special exposure meters are designed for use with flash; you can’t use an ordinary exposure meter to meas.
The most realistic and usually most pleasing lighting resembles daylight, the light we see most often: one main source of light from above creating a single set of shadows. Lighting seems unrealistic (though there may be times when you will want that) if it comes from below or if it comes from two or more equally strong sources that produce shadows going in different directions.
Flash can be used in conjunction with ambient (already existing) light. It can be very useful as a secondary light source to fill in the shadows of natural light, without losing the basic light & shade patterns nor the general atmosphere.