CSS backgrounds add styling to the background of the page. This can be done by adding an image or by adding a color.
The site uses a blue background color in the header and a cream background color in the body of the page.
This box uses an image for the background.
This box uses a repeated image.
You can repeate horizontally or vertically
CSS Tricks explains background images in greater detail and W3Schools is a good place to get the basics.