I'm going to assume you're actually talking about viewing pictures on the monitor on the back of the camera and not in the viewfinder. If I'm wrong, please post a comment and say so.

You're looking at blown-out highlights. These are the portions of your pictures where you've lost all detail due to overexposure. Those portions of the picture are solid white, with no shading. That's not necessarily a bad thing, though often it is. You can correct this by using exposure compensation to reduce exposure.

That was the long answer. Here's the short answer: Press up/down on the multiselector to cycle through different views of your picture.