It's how the camera determines the best exposure.
Normally, there are 3 types of metering methods:
- Multi zone / matrix metering
Camera evaluates mutiple area of the scene and decides on the best exposure. How many areas and it's position vary according to camera. Normally the most accurate metering method.
When to use: Multi area metering should be the default setting on your camera and works best most of the time.
- Spot metering
Camera looks at a small (1-5%) area to decide on the best exposure and not influenced by other areas.
When to use: When you wanted the subject to be properly exposed. E.g: A back lit subject may be underexposed using other metering methods, but by using spot metering, the subject is properly exposed while the background is overexposed. May be useful also in macro photography.
- Center weighted metering
Camera looks at the center of the frame (60-80%) and decides on the best exposure, giving importance to the center and less at the edges. Reputated to be the least accurate metering method.
When to use: May be used when taking portraits, or when you wanted to ensure the center of your frame to be properly exposed in tricky lighting conditions.