Your camera uses ‘metering’ to determine what the correct aperture and shutter speed should be depending on the amount of light that comes into the camera and the ISO.
The three main metering modes on digital cameras are:
Matrix metering – Nikon and Evaluative metering (Canon)
Use this for most of your photography
Centre weighted metering
Evaluates the light in the middle of your frame
Evaluates the light in a very small area of your frame based on where you put your focus point
Here’s three very useful videos that will help you to understand what metering modes are and how to use them: