Drag the dot across a hillside seen from above. The two inputs, x and y, pick a spot, and the output is its height: brighter means higher.
Think of the map as a hillside seen from above. Bright means high ground, dark means low ground. Drag the white dot, or use the sliders. One input gave you a curve; two inputs give you a whole landscape, and every (x, y) spot has its own height.