a behavior of windowing systems, available in various window managers for X Windows (like Sawfish and WindowMaker).
autorais means that a partially overlaid window will "raise" itself to the top (and usually become completely visible) if your mouse pointer passes over it.
Some people like to configure an autoraise delay. Other people absolutely hate autoraise.
At least for me, autoraise is the best buddy of "focus follow mouse". The combination makes for minimal clicking.