This is a graph of the level curves (in the x-y plane) of the function f(x,y) = -(x^2+y^2-1)^2. (Ignore the numbers on the sides; the point (0,0) is in the middle of this plot.)

Notice that the dark red represents the highest values of the function (f(x,y) = 0 is the maximum) and the lighter shades of red, going to orange, yellow, and then blue, represent the function getting smaller (more negative). Does this diagram help you visualize the 3-D plot of the surface z= f(x,y)?