Aperture, or f/stop, is the lens diaphragm opening inside a photographic lens, and the size of the opening regulates the amount of light that passes through onto the film inside the camera the moment the shutter curtain in your camera opens during an exposure process. It also affects how much of the scene, from foreground to background, is in sharp focus.
Your camera has an automatic setting called the Aperture Priority Mode (A or Av). When you pick this mode, the camera automatically chooses the appropriate shutter speed for proper exposure. If the depth of field (how much of the scene is in acceptably sharp focus from foreground to background) is most important for your photo, you may want to select your preferred aperture using this exposure mode on your camera. (You also have the option of choosing Manual mode on your camera, which will allow you to choose the aperture and shutter speed if you want complete control). If you want to emphasize your subject by placing the background out of focus, select a large f/stop (small f/number) such as f/2.8 or f/4.0 for example. Select a small f/stop (high f/number), like f/11 or f/16, to capture more of your scene in focus.
Using an aperture of f/5.0 allowed me to focus in on the tiny details of this newborn's precious little feet.
By using an aperture of f/5.6 to put the older brother out of focus, the story of this picture then becomes about the younger brother.
I was able to focus in on this piece of grass, isolating it from the background, by using an aperture of f/1.8.
The photos below were taken at three different apertures: f/4.5 (top), f/11.0 (middle), and f/29.0 (bottom). Notice how the background changes from blurry with the aperture wide open to much sharper and more distracting with a very small aperture.
Next time you are photographing your subject consider what aperture you would want to use to portray the best story. Do you want to isolate your subject from the background, making him/her the focus of your image? Or do you want to capture the whole scene in perfect sharpness? Play around with different apertures (as well as different aperture / shutter speed combinations), and see the various photographic masterpieces you can create.