The bottom part of the diagram shows some contour lines with a straight line running through the location of the maximum value. The curve at the top represents the values along that straight line.
A two-dimensional contour graph of the three-dimensional surface in the above picture.
Contour lines or isolines are used when plotting a function. All the points where the function has the same value are connected. Two well-known examples, where such lines are commonly are height lines on topographicalmaps, and showing areas with the same pressure or temperature on weather charts. Contour lines are an application of level sets.