Your camera has several different methods of calculating the correct exposure for a given image, and understanding how each of them works is crucial to getting properly exposed photos. This helpful video will help you understand metering modes.
Coming to you from Saurav Sinha, this great video talks about metering modes and their impact on your imagery. Most cameras have several metering methods built in, and if you're shooting in a mode in which the camera calculates any part of the exposure, it's important to choose the correct metering mode for your subject matter. For example, when shooting baseball, I know that I need a fast shutter speed and as wide an aperture as possible, but with as fast as the ball moves, I don't have time to change ISOs as it darts in and out of shadows and light areas. So, I typically use spot metering tied to my autofocus point and an exposure compensation of -2/3 stops to protect the highlights. Using another metering mode would blow out the images because the camera would pick up on the relatively dark crowd behind the players, who are lit by powerful light on top of the stadium. Check out the video above for the full rundown.