By "gradient" they mean a color gradient, that is, a gradual transition from one color to another across a two-dimensional region.
When they say "slowly" and "as it moves", they don't mean animation; the image itself will be static. You can think of it as meaning that if your eye were to move across the region from one side to the other, you would slowly see the color change from dark to light.
That is — they want something like this: