When you look through the view finder and press the button half way down your camera will make an exposure reading (as well as start to focus). In short it will look how much light is there available, this is called the EV (exposure value) and will be the basis for your photo depending on how you set your camera.Your Alpha has 3 metering modes
from the A900 handbook
Your camera is only measuring the light that it can see, and may be fooled into measuring incorrectly in a variety of different lighting situations. The EV that the camera measures is also only a guideline based on the decisions made my an engineer in a lab somewhere, and my not be the correct value for the image you want to make.
The 3 aspects (settings) of your exposure are all in your control and changing any one will not only have effect the final image but also one or both of the other two settings e.g. I like to shoot with my cameras in A mode. I set the f/ (aperture) to the d.o.f that I want and I chose the ISO to match the lighting conditions (dependant on approximately what speed I want to shoot at) . If I adjust my f/ and leave the ISO where it was my camera will automatically compensate by adjusting the shutter speed. Remember you are taking the photo not the camera.
There are two great tools built into your camera to help you check and adjust your EV, the Histogram and the EV compensation both of which if used correctly will save you a lot of time latter when you are correcting/editing your images. Using the histogram can be overwhelming at first but is something that every photographer should understand to get the best results from their camera. It is also to much to go into right now so Ill cover it as a separate topic in the future ( but if you want to read something now look at luminous-landscape they have a great article that can probably emplane it better than I can any way 🙂 )