Tips About Aperture-Priority Auto on nikon d3000?

What your talking about is known as depth of field. Aperture is confusing in that small aperture means big f/stop ex. F/22 and most of your picture will be in focus. A larger aperture f/2.8 for example means that the image your trying to capture will be in focus and the background will be out of focus. If this doesn't help look up DOF or aperture on google.