SLR = Single Lens Reflex – This refers to how your camera differs from a point and shoot. With an SLR, the image you see through the viewfinder is reflected with mirrors onto the chip inside. What you see is what you get … because the lens sees the same thing. With a point and shoot, the viewfinder doesn’t reflect back any image with mirrors…in fact, what you see is not what the lens sees because the viewfinder and lens are not connected in any way. The viewfinder is just a hole in your camera that sits above the lens. This can be a problem, especially if you are close to your subject. For example, have you ever taken a picture with a point and shoot and later saw that someone’s head was cut off? What you see is not what you get with point and shoots. However, your LCD screen will be accurate for both camera types.
Above: Here's a picture of what your camera meter reading looks like when it's properly exposing a picture. The largest green line is right smack in the middle. You can either see it by looking into your viewfinder (it's down at the bottom usually) or it is usually on your LCD screen.
To overexpose, it goes to the right. To underexpose, it goes to the left. The numbers are your shutter speed, f-stop, ISO speed, and picture number.