Each camera has a photometer, it is like a sensor that detects light. What you can adjust with changing the metering mode, is the area that the sensor will cover.
So lets see which are these modes and how they will affect the exposure reading, and your photo.
This mode basically lets the sensor take a sample from the whole frame, make an average calculation of the light, and give the result. This can work really well for landscapes or pictures that have consistent lighting across the whole range of the frame.
This mode makes the sensor detect light only by a single spot, one that sometimes coincides with the focus point (if you have a single-point focus mode enabled). Basically, this will be the center point of your frame. This can help if you have a high-contrast scene—for example, a face on a brightly lit day and you want to properly expose only the face. This will help you isolate the subject and get a proper, more accurate metering and then adjust your setting based on that, with better results.
This is basically a wider version of the spot mode. It lets the sensor detect light from a wider area around the center spot, giving you an average calculation for your metering.
It is important to know that you can adjust the exposure mode based on the conditions of the shooting. This will save you time from having to take the picture again because you overexposed the sky or because the bride’s features look too dark. You can use it to your advantage and make your pictures look great with less effort and frustration.