Metering is how your camera reads light to determine what the “correct” exposure should be. There are several different Metering modes; Center-weighted, spot metering & multi zone to name a few different modes. The most common and original metering mode is Center-Weighted. Center-Weighted metering take in all light in your sensor  and takes an average, over all this is a good system if your subject is evenly lit. This however doesn’t work well if there  is direct sunlight and shade in the same picture,  your image will be over or under exposed based on the situation. So with this system you would take a meter reading then use your photography super knowledge to correct your exposure. 

Spot metering is my favorite by a long shot. With spot metering you take readings from small spots in your view finder vs your full view finder. So let’s say your subject is in the shade on a sunny day you can put your focus point on your subject and the camera will make a reading off that spot. When this pattern is used properly it is the most accurate.

No matter what meter pattern you use  its not 100% accurate as you will always have to evaluate the environment and adjust your exposure.