I'm assuming you mean when you are previewing the image on the LCD?
The black/white flashing areas are showing you where the highlights are blown (where the image is over-exposed and all detail has been lost). You can turn the feature no/off in the camera menu if you prefer.
If you are blowing the highlights, reduce your exposure accordingly - smaller aperture / higher shutter speed / lower ISO / exposure compensation / filter etc., depending on your subject of course.