a photograph's exposure determines how light or dark an image will appear when it's been captured by your camera this is determined by three camera settings aperture, shutter speed, iso (exposure triangle)
shutter speed is the amount of time the shutter is open. In most cases you would probably be using shutter speeds of 1/60 or faster. If you're using a slow shutter speed (anything slower than 1/60) you would need a trypod. If you need to take a picture of something moving you would need a fast shutter speed. If it is dark you would be using a slow shutter speed.
Aperture is the opening of the lens. Aperture has a big impact of depth of field. Large aperture will decrease depth of field while small aperture will give you larger depth of field. The aperture that you have set impacts the size of that hole. The larger the hole the more light that gets the smaller the hole the less light
iso measures the sensitivity of the image sensor. Higher Iso are generally used in darker situations. When you use high iso its alot more grainy. Situations you might need to use high iso sports events, museums, concerts
the reason we adjust white balance is to get the colors in your images as accurate as possible.
Aperture Shutter Speed and ISO, Photography 101