This is the section in which you will learn how to graph functions of two variables in a two-dimensional coordinate system and how to read such graphs.  This is a very small section, however, a very in-depth one.  There are also many real-life applications.  For example, you can draw a map of the continental US and map isobars of barometric pressure.  This may not be real useful to the average person, but is extremely helpful to professionals such as meteorologists.