No doubt, you've heard it said that you should shoot on overcast days, or wait for the sun to pass behind a cloud. And you know that you can get better results when you bounce your camera flash or use an adapter that diffuses the light.
These techniques are all leveraging what you might refer to as the First Law of Lighting: A broad or large light source tends to create a soft, more attractive light on your subject. A narrow light beam, such as from your flash or the sun (which is admittedly much larger than the earth, but narrow from the camera's perspective) is hard light that makes for sharp shadows and high contrast. But if you diffuse that light over a wider area--making it big and broad--you can soften the light and reduce the contrast in a way that's usually more pleasing to the eye. Knowing (and applying) this one fact about light allows you to transform your photos, because now you know that diffusing or broadening narrow light sources will generally de-harshen your photos, ease shadows, and make subjects look more attractive.
That might sound obvious, but let me take you back to science class.