All light meters, whether handheld or on-board, are designed to examine a scene and calculate the amount of light needed to produce an 18% gray image. This is a magical number devised years ago to render (in a perfect situation) all the tones from pure white to absolute black. Same for color except colors are perceived as grayscale. Problem is there is no perfect situation. If you are looking at a scene with more white or highlights then the meter will adjust the settings to lower the light to reduce the overall scene to 18% gray, thereby darkening the shadows and losing detail in those areas. If you are shooting a particularly dark scene with a few highlights, the meter will suggest more light/slower speed to increase the detail in the shadows but this may blow out the highlight areas. A spot meter allows you to meter individual areas of a scene. Often, I will meter on different areas of a scene, and select a manual setting in between the high and low. If in doubt, I bracket my shots. Hope this helps some...
Dave
P.S. Pick up an 18% gray card at a photo lab or order one online. Carry a piece of it with you on shoots. Using the same incident light as your subject, meter the gray card, and use that setting. Perfect exposure every time.