Lighting is a massive subject.  I will try to break it down to easier to digest pieces.

Some situations you only get to use natural, available light.  Other situations you can add your own light, either by reflecting/bouncing natural light into the scene, or adding artificial lighting through the use of … whatever you have.

Critical considerations come down to:

1) Color of the light

2) Quality of the light

The color of the light is general referred to as temperature.  Cool is bluish, warm is golden, daylight is pretty much white.  If you add a flash, daylight, to an otherwise warmly lit scene or subject in a room, the natural warm light will look completely different, so you normally (unless you are trying to achieve the odd look) want to change the color of your flash to match the native lighting.  This is done using gels attached to the flash head that alter the color.

The quality of the light is a masters degree in art by itself, but basically it refers to the shadows cast by the light.  A small relative light source will create a very sharp harsh shadow, while a wide light source will create a very diffuse shadow.  There are numerous styles in various painter’s techniques through the centuries.  These can all be replicated in photography.  The painting masters are considered the examples to mimic for good reason, their art is timeless, so they make a great guide for our photographic efforts.

More detailed explanations will be covered in separate articles, mainly just know the basics, that lighting is more than just adding more light, you control the color, direction, intensity, and relative size of the light source, take full advantage and your pictures will look far better very quickly.