Knowing how your digital camera meters light is critical for achieving consistent and accurate exposures. Metering is the brains behind how your camera determines the shutter speed and aperture, based on lighting conditions and ISO speed. Metering options often include partial, evaluative zone or matrix, center-weighted and spot metering. Each of these have subject lighting conditions for which they excel-- and for which they fail. Understanding these can improve one's photographic intuition for how a camera measures light.
Understanding light metering system is one of the most needable knowledges in digital photography cause it gives you a starting point for determining exposure settings in different lighting situation.