Photography is all about light, the amount of light or not, hitting the film, or sensor. The proper control of that light can make the difference between a bad photograph and a great photograph.
Camera settings, shutter speed, aperture and ISO all alter the amount of light directly reaching the sensor or film but that isn't the only way to control the light in a scene.
You might have natural light alone, or artificial light or a mix of the two, you can use reflectors, either purpose made reflectors or something in the scene such as a wall, or even the ground, you might be able to carefully position the subject to make the best use of the available light, or you can use additional light, either flash, or fixed lights, and then either on camera or off camera, to supplement the available light.
Though todays digital cameras are increasingly good in 'automatic' mode, where the camera will sense the amount of light and shade in the scene and make a 'guess' at what settings are then best to use, it is still only 'guessing' and there are frequently times when either it gets things not quite right, or there is a situation where it simply can't cope.
Take this scene. A sofa in front of a window. A fairly typical 'indoor' scene, but you can get a similar situation outdoors, with a mix of light and shade.
In this instance, the sofa is in too much shade, and yet there is too much light coming through the windows. If you let more light reach the sensor to properly expose the sofa, then the windows will be overexposed, if you reduce the light reaching the sensor to ensure the windows are properly exposed, then the sofa will be in far too much shadow.
This sort of situation is common, both indoors and, in particular, outdoors, and is perhaps what many people most struggle with.
No matter what you do with the camera settings you won't achieve a perfect shot.
So what can you do?
You could reduce the amount of light coming through the windows, by closing the blinds, and then expose properly for the sofa. If you had this situation outdoors, then you could move the subject into an area of shade and then expose for the subject, but perhaps you want the window light or light in an outdoor scene. You then have to add light to the overly dark area, you could switch on indoor lights, though this may not be the best solution, you could use reflectors to reflect the light coming through the window, or you can use fixed lights or flash. In this instance direct flash, whether on camera or off camera might result in an overly harsh light and shadows, so using reflected or bounced light is the best solution, even when using flash to provide that light.
This is the result of using studio flash, but directed at the ceiling rather than directly at the sofa, the light then bounces off the ceiling providing a fill light to light the sofa so that exposure for the window light can then be properly balanced.
If you are outside, you can bounce light off the ground or a wall or use a direct reflector or else a diffused flash (where a cover softens the light of the bare flash) to achieve a similar effect.
As ever with digital cameras, you can readily spend some time experimenting, just trying different things out and then instantly seeing the results.