This goes back to the lesson about f/stop and depth of field. In order to keep the subject of your picture in focus and to get the background out of focus, then you need a shallow depth of field. Basically, that means your camera is not reaching very far to choose what to focus on and it will only focus on what is close to it. To get a shallow depth of field, you need a very wide aperture, which will be indicated by a low f/stop number.
That sounds confusing, but just remember that depth of field is the opposite of what it may sound like. A wide aperture equals a low f/stop and a narrow aperture equals a higher f/stop.