By good, you of course mean a small depth of field
The F stop is how open the aperture of the lense is. Crazily, the smaller the number, the more open the aperture is, and also the smaller the number, the larger the f-stop actually is. More info here - http://en.wikipedia.org/wiki/F-number. The more open the lense, the less depth of field you have, or put simply, the less that will be in focus in front or behind the bit you focused on. Best thing to do is to take pics of something against a background that is a distance away, say a few cups against a wall a few feet away that has a pattern. Distance them out and then take photos of them with varying apertures and see what varying them does. Large aperture (small f-value) will result in very little being in focus. Small aperture (large number) will result in more being in focus