You've probably seen beautiful photos of flowers which are close to the camera and in crisp focus, while the background is soft and fuzzy. That is the result of a camera's limited depth of field. Depth of field can range from extremely narrow (for example, a photo of a flower in which only one petal is in focus while the rest is out of focus) to effectively infinite (such as landscape photos where everything in the image is in crisp focus).
Depth of field is primarily affected by the camera's aperture setting. As we explained above, a larger aperture (which has a smaller f-stop number) will give you a narrow depth of field, while smaller apertures (with larger f-stop numbers) will result in a large depth of field. Depth of field is also strongly affected by focal distance, which reflects both the kind of lens you're using and how close the subject is to the camera. Closer subjects will have narrower depth of field, while distant subjects can have nearly infinite depth of field. Calculating depth of field is actually a complicated business involving something called the Circle of Confusion. If you're not interested in the math, just experiment with your camera and a variety of f-stop settings and subjects to see how you can manipulate depth of field.