MMatrix Metering
An advanced metering mode in which the camera's computer sets exposure based on its analysis of the scene's components. It is generally regarded as the most accurate metering for most lighting situations, including those with complex lighting. Matrix Meter or 3D RGB Color Matrix Meter gathers information from the red, green, and blue sensors and factors in distance information provided by the lens as it evaluates proper exposure calculation. This meter instantly analyzes a scene's overall brightness, contrast, and other lighting characteristics, comparing what is sees against an onboard database of over 30,000 images for unsurpassed exposure accuracy, even in the most challenging photographic situations. By the time the 3D Matrix meter has made its considerations of colors by hue and saturation, tonal ranges by brightest and darkest, areas of similar tonality that are connected or separated, distance to the subject, and compared that to its database generated from photographic images, it's got a very good idea of what the exposure should be.
If you are a beginner, Matrix is where you should start. As your skills grow, and they will, you will acquire a better understand of when it might be beneficial to use other light metering options.
What is the database of over 30,000 images? Over the years Nikon has studied the color, area of coverage, focus distance, contrast, size and shape of shadows and highlights and exposure characteristics of over 30,000 actual photographic images and incorporated this data as a reference source for the expert exposure system that is the 3D Color Matrix Meter.