Help me understand a 3d graph I've just seen this graph and while it's isn't the first 3d graph I've seen, as
a math "noob" I never thought how these graphs are plotted. I can draw 2d graphs on paper by marking the input and output values of a function. It's also easy for me to visualize what the graph I'm seeing says about the function but what about graphs for functions with 2 variables? How do I approach drawing and understanding the visualization? 

An illustration should make things more clear. Here is the function you mentioned:

I have drawn contours (the black curves) to indicate the set of points for which the function is equal to a particular constant. As you go up the "wall" from one contour "rung" to the next, the value of the function (the z axis) increases.
