Backgrounds In CSS: Everything You Need To Know - CSS Essentials - page 9

As we scroll down the page, the background scrolls up until it disappears.
But when we set the background-attachment to fixed , we are telling
the browser that when the user scrolls down the page, the background
should stay fixed where it is — i.e. not scroll with the content.
Let's illustrate this with another example:
background-image: url(test-image.jpg);
background-position: 0 100%;
background-repeat: no-repeat;
background-attachment: fixed;