The word fixed, when discussing backgrounds in html/css is used to describe how the background moves in relation to the page.
There are 2 options - fixed and scroll.
scroll - (default) The background image moves when the rest of the page scrolls
fixed - The background image does not move when the rest of the page scrolls
To add a background image and have it fixed, use:
background-image: url(your_file.gif);
background-attachment: fixed