There are two classes in the scatter plot -- blue and red points. These can be separated by a line. A line has two parameters, for example, a slope and an inter

There are two classes in the scatter plot -- blue and red points. These can be separated by a line. A line has two parameters, for example, a slope and an intercept (although, other parameterizations are possible). These line parameters are the x and y axis. The lighter and darker areas of the heat map indicate better and worse lines, respectively, for separating the two classes of points.