When you look through your view finder in your camera (the eye piece) you should see something similar to this scale. This is your light meter and it is essential to understand how it works. Read the articles in the links for this post to learn about it.
Here is a depth of field demo I did using cans I had in my kitchen. They images go from f18 to f2.2. Notice at f18 everything is in focus. As f-stop increase through to f2.2 what do you notice about the cans that are in focus and background?
Look at your eyes in a mirror with all the lights on. While still staring at your eyes reach over and turn out the lights. What happens to your eyes?
Your eyes' aperture is adjusting to let more or less light in depending on how bright your surroundings are. They do this with out you noticing so that you don't go blind on a bright day or walk into a pole in the evening. Your camera also has an aperture and can do the same thing but you have to tell it to by setting the aperture. Wider opening in the aperture = more light coming in ( f/1.4 ) Smaller opening in the aperture = less light coming in ( f/16 )
See the diagram on the left. The "f" stands for F-stop which is a unit of measurement used in photography telling you the size of the aperture opening. The numbers are backwards, f/1.4 means a large opening, f/16 means small.
The aperture setting also controls your lens' depth of field which means the areas of your image that will be in focus. This has a huge impact on your image. Read through this article to understand it...
How aperture and depth of field work