Metering is how a camera decides what exposure (how much light is let in) to use and it adjusts the settings it uses according to this to get a shot that’s correctly exposed. However, the results that are produced aren’t always desirable and this is when picking one of the metering modes available comes in-handy.
Generally, there are three metering modes to select from: spot, centre-weighted or multi-pattern.
As spot suggests, this is where the camera will only use a small area of the scene to help determine how much light is needed for the photograph. This is usually the centre of the image but some cameras let you use a spot off-centre. This mode is good for portraits where the sun is facing the camera. Usually, the subject will appear darker/silhouetted against the bright background but if you line the spot so the camera meters off their face it should stop this from happening. Depending on the camera, spot metering uses between 2-4% of the centre of the frame.
It's also useful for shooting portraits as you always want the eyes to be sharp and this metering mode will give you the precise control you need to ensure this happens.
Sometimes the spot area your camera uses will be shown in more detail in the camera manual.
You can think of center-weighted metering as a vignette – Even though everything in the image is taken into consideration, prominence is given to the centre of the image and priority gets less and less towards the edges.
This is where you camera tries to look at everything in frame, assessing how many light and dark areas there are (as in the shot below) then it take an average of all the light that's available to try and produce a scene that's exposed correctly. This is usually the default setting for most cameras and how the average is calculated changes from camera to camera. It's good for scenes where contrast is low but it can struggle when there's a big difference in contrast such as a really bright sky sat against a darker landscape.