I first have to preface this by mentioning that not all cameras have this feature. Before you tell me your camera doesn't have this feature do yourself a favor. Look in your manual for "exposure lock" to make sure (just in case). Some cameras also allow you to customize button assignment. Typically the button is placed somewhere near the shutter release and is marked with an asterisk (*).
Exposure lock only works if you are using your camera in one of the auto modes (aperture priority, shutter priority, program, etc.) It will not work in manual since you control exposure instead of the camera.
In a typical setup, the camera's lens is used for both focusing and metering, based on the type of focusing and metering mode you select. Pressing the shutter button half way activates both the auto focus and the built-in meter. So by default, whatever you're focusing on is what you are also metering. However, not all lighting situations work with this set up. Specially high contrast scenes where there are heavy shadows. Or scenes where there are overly bright or overly dark backgrounds.
To best use this function set your camera to spot or center weighted metering. That way you can use a small section of the scene as your meter target rather than a larger zone, evaluative or matrix spread.
If you notice the light hitting the building you can see that it was a bright, contrasty day. I am interested in the building so I wouldn't bother metering the sky since that would underexpose the building. The building, however, has mixed light falling on it. This leaves me two choices, meter the light or meter the shadow. Since digital cameras handle shadows better than highlights I chose to meter the light area of the building. In this case I actually metered on the gray roof then made some adjustments in Lightroom in post. If I had metered the shadows I would have overexposed the light hitting the roof and center area, loosing detail there. So keep this in mind, you have more leeway with shadows and can regain that information in post processing but overexposed images that have gone completely white can never be recovered.
Here is another commonly found problem, shadows under hats. These two guys were on the Waterbury green on a shady day, beautiful for street photography. They saw me with the camera and allowed me to take some shots of them. Since I wanted to see their faces (though one was a little shy) I needed to make sure my exposure was proper for the shadows under the hats. If you have your camera set to spot metering and you're close enough to your subject the process is easy. Aim at the eyes and shoot. However I was a bit further ways with a 400mm lens. Instead I metered the shadow under in the armpit area. You can see that I lost some information in the white hat but that's okay since the hat wasn't all that important.
With practice, using the FE Lock button will become second nature and you then won't have to compromise because of your focus.