When you want to make your subject stand out as a focal point by blurring out the background, you are looking at adjusting the depth of field. This can be done by changing the aperture, or f-stop, on the camera, which controls how much light is let into the camera. When the f-stop is set at a lower number, the opening is larger, which means more light can get in, and vice-versa. To create a feeling of deeper space by blurring out the background or to ensure your main subject stands out, you want your aperture to be wider and set to a lower number (say around f/5.6). However, to bring everything closer together by leaving most of the image in focus, you want the aperture to be smaller by setting it to a larger number (f/16 for example).
A wider aperture (pictured left) lets in more light and is often accompanied by a lower f-stop, while a smaller aperture (pictured right) lets in less light and is accompanied by a higher f-stop.