The light metering system built into your Nikon is complex and sophisticated, but it’s still not foolproof. That’s why your Nikon has a choice of metering patterns.
By default, Nikon digital cameras use so-called ‘Matrix’ metering. This splits the scene into different zones that are measured individually. The camera then builds up a picture of the distribution of light in the scene and checks this against an internal ‘database’ to try to work out what kind of subject you’re shooting and the exposure that will give the best result. It sounds really clever – and it is – but ultimately the camera can only guess at your intentions. If you’re just starting out in photography, Matrix metering will probably deliver better results than you could work out for yourself, but as you gain experience you will encounter more and more situations where you need to take control.