In a camera, the metering mode is what determines the exposure for the image.  Most cameras by default are set to a standard evaluative metering mode that takes the overall exposure of the image and trys to make it as even as possible.   In some situations this won't work!

(Matrix/Evaluative Metering Mode)Learning how your metering modes work, and when you will need to change them is very important to capturing great exposures.  Most cameras will have three different metering modes.  One is the overall metering mode (matrix mode on Nikon, evaluative metering mode on Canon.)  This mode takes account all the light entering the image and trys to find the most balanced level for everything.  If your scene is fairly even (mostly bright, or mostly dark) then this mode will do a great job.  Most of the time, this is the mode you will be in, and by using a little bit of simple exposure compensation, can be the only mode you need to use.
(Matrix/Evaluative Metering Mode used on image to left, Spot/Partial Metering used on athlete for image to the right)The next mode that works well for action is spot metering/partial metering mode. This mode will evaluate a very small part of the image right where you set your focus point, or a tiny spot right in the middle depending on your camera and menu settings.  This way, if the scene is overall dark, and your subject is bright, you can meter off the subject by using spot meter mode.  Since your subject is the focus point (make sure your camera has this feature), the camera will evaluate them to a perfect exposure.  This will keep the overall scene dark, and the subject perfectly lit like you intended.  Had you tried taking the same shot with Matrix/Evaluative mode, the subject would be overly bright, and the scene would be brightened up, losing the dramatic look.
(Spot/Partial Metering Mode used on athlete to brighten face/jersey)Finally, there is the center-weighted mode, or center-weighted-average mode.  Center-weighted mode works by taking a sample fo the center and some area around the middle for an exposure.  It's larger than spot-metering mode, but smaller than Matrix/Evaluative mode.  This mode could be particularly useful for in-water surfing images when you need the surfer to be well-exposed and aren't able to look through the camera viewfinder so you aim dead center.  Try this mode when using matrix won't work, and you don't need to be as specific as spot metering.Learning your metering modes can really help you understand how your camera works, and why it comes up with the exposure it does.  Try testing each mode on a stationary object to see how the effects work before changing in the field.  For the most part, matrix or evaluative mode will be your best bet for almost all situations.