This is a system used to measure the lighting in the scene so as to determine the correct exposure settings. There are four modes available, which work in slightly different ways.
Different types of metering include:
Evaluative metering meters the overall scene in multiple sections in the viewfinder and combines the results to find the correct exposure based on the subject's position and lighting conditions. This is the default metering on most DSLR cameras
and can be used for the majority of your photography.
Centre-weighted average metering concentrates the metering on the central part of the viewfinder, but also takes the rest of the frame in to consideration. This mode is favourable when you want to capture a well-balanced exposure, but biased towards whatever is right in the middle of the shot.
Partial metering weights the metering on the central area of the frame only, usually around 6% of the frame. It’s the best option for where a central subject has considerably different lighting to the rest of the scene, for example in backlit situations.
Spot metering is similar to partial metering, but covers a much smaller area of the scene, usually less that 2% and right in the middle of the frame. This mode is useful for where you want to pick out a small subject.