Okay, what you have there is veiling flare. Have you got a
lens hood? If so use it. Shooting directed towards the sun can produce stunning results, but you become more susceptible to flare, especially if you shoot with zoom
lenses. you shot above is a great example of an image that you would have done well to use an ND grad filter with (it's like putting sunglasses over the the bright part of the image, in this case from the horizon up).
A highlight is the brightest part of the image that retains detail. A cloud may be a highlight in an image. If you blow the highlight, it means that you have exceeded the dynamic range of the camera with your chosen
exposure, and you have lost detail in the cloud (pure white). Sometimes this is unavoidable, especially if your subject is in much darker
light than the cloud. You then need to decide what is more important in the image; a well exposed subject, or an image without blown highlights which may be able to be fixed in the darkroom or on the computer. So sometimes it is a compromise. Landscapes can have a large dynamic range (range of luminance from dark to
light), and ND graduated filters are often employed to narrow the dynamic range that the camera has to capture. Solid state capture doesn't have the ability to capture as much of that dynamic range as ag-x capture, in camera, and this is one of the compelling reasons why 80% of landscape photographers use film. But digital has the ability to make up some lost ground in software such as Lightroom or Photoshop, where editing a
raw file, within reason, can be done without too much detriment to the final image. And you will get instant feedback on where you highlights and shadows are falling via one of the two methods outlined below.
An
exposure is the action of your camera making the photograph. It is exposing
light to the digital sensor, through the
lens. So to make a correct
exposure, you need to establish what size
aperture you want to use, and what
shutter speed to use with your chosen
ISO. There should be plenty of info here in the Beginner's section.
On you camera, you should have the function for the camera to display blown highlights or blocked shadows. I have no idea what this display option would be, but when you review an image that you have just taken on the LCD, this function will tell your camera to flash parts of the image where you have lost detail. If you have a very bright section in your image, the screen may flash red over that section, and if you have really dark spots, it may flash blue.
DSC_0792.jpg
You probably want to avoid blown highlights (the red section) in most shots, and I can't think of any landscape image where blown highlights are welcome. So in the above shot, you can either go the filter route (preferred method), or expose for the highlights (in this case, underexpose your shot so you don't see any red flashing on your LCD) and bring the foreground up in Lightroom or similar. Your shot here is not fixable, as once you lose your highlights, they are gone for good. Blocked shadows don't look as bad (at least to me) as blown highlights. I will purposely block up the shadows a tad in colour images, but sometimes in black and white images, I will block them up a quite a bit more. That's a personal choice, but perhaps start by trying to capture images within the dynamic range capabilities of your camera. I don't mind blown highlights on things like a motorbike where you make have a specular highlight on some chrome; pretty much, if my eye can't see the detail, then I don't try to capture that detail in the image either.
Another way of establishing where your highlights and shadows are falling is with the histogram. There should be enough references in the beginner's section for you to study.