What is "Configurable clipping rectangles"?
Ignacio's answer is technically correct, but I suspect it doesn't help you much.

If you think about drawing a 3D landscape, it should be pretty clear that you don't need to draw the entire world. All you need to draw is the portion which will be displayed. And the area which needs to be drawn is typically a rectangle - most obviously the edges of the monitor which will show the final image. So a clipping rectangle is just the description of the edges of the displayed scene. The rest of the scene, outside the borders of the rectangle, are "clipped" and no processor time is wasted working on that area. In the case of your accelerator, the exact parameters of the rectangle can be set in software, and so are "configurable".

See http://en.wikipedia.org/wiki/Clipping_(computer_graphics) for more details.