Your aperture (how large the shutter opening is) controls your depth of field and how much light enters the camera at one time. The larger your aperture, the more light and the narrower your DoF. The smaller your aperture, the less light, but the larger your depth of field.
The aperture setting can be confusing because most dSLRs simply display the f-number, and the instinctive reasoning is that a larger number means a larger opening. But actually, the aperture setting is given as a ratio of f/# (f=focal length, #=f-number), so f/8 is smaller than f/2, just as 1/8 is smaller than 1/2.
However. There's one more gotcha, with a dSLR. You're used to thinking of the viewfinder as a WYSIWYG way of composing your photo. It actually isn't in two very specific ways. The first is that the area covered by the viewfinder on the XT is actually smaller than the actual image captured (i.e., your edges aren't actually where they are). The second is that the depth-of-field you see isn't necessarily the depth-of-field you're going to get.
The camera leaves the aperture at its maximum setting when it isn't taking a picture. The reason for this is that you'll get the most light that way to see by. It helps the autofocus system as well. The lens is only set to the aperture setting you choose just before the picture is taken. So, when you look through the viewfinder, you're seeing the minimum depth of field that lens can give you. Which is why Canon also put a DoF Preview button on your camera.
If you've stopped down the lens to get more depth of field, you'll only see it in the viewfinder, if you hold down the DoF Preview button. The lens will then stop down to what you set, and you can see if everything's going to be in focus or not. But because the opening got smaller, the view will get darker, which may make it hard to see detail, especially at tiny apertures like f/22. But it can be useful for checking that you've got the depth of field you want before taking the photo.
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