When you're looking through your viewfinder, the aperture is wide open (say f/4).
The background will probably be out-of-focus, because there is little depth-of-field at f/4.
Therefore, you won't pay much attention to the background.
When your press the shutter release, the aperture will change to the one needed for a good exposure.
The aperture will probably get smaller, and more will be in focus, due to greater depth-of-field.
Later, when looking at the photograph, you may ask:
Why didn't I see that telephone pole coming out of Uncle Fenster's head?
You didn't see the telephone pole.
It was out-of-focus, because the image in the viewfinder had little depth-of-field.
Use the depth-of-field preview button if your camera has this feature.
When first trying this feature, students often say they can't see anything.
The image is too dark.
Keep using the feature, and you'll learn how to see what's in focus, despite the darkness.