Just like focus lock, most cameras have an exposure lock feature. When using an automatic or semi-automatic mode the camera uses light information from the entire frame to decide how to expose the photo (there are actually several different exposure modes that a camera can use, which will be covered later, and exposure lock works with all of them). If you wanted to take a photo of someone sitting in shade with a bright blue sky behind them the camera will see all of the brightness and set the exposure for that, leaving your actual subject under exposed. If you use exposure lock however you can set the exposure for the subject directly, ensuring that the important part of the photo is well exposed.
Like the focus lock this is usually done by pointing the camera at the area that you want well exposed, half depressing the shutter button, then composing the photograph and pressing the button in completely. Most cameras will lock the focus and the exposure at the same time.