Contour plotting is a powerful technique for visualizing data in two or three dimensions. It is used to represent the shape of a surface by plotting a series of contours, or lines of equal value, on a graph. Contour plotting can be used to identify trends in data
From Wikipedia
A contour line of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value to the state. It is a plane section of the three-dimensional graph of the function parallel to the -plane. More generally, a contour line for a function of two variables is a curve connecting points where the function has the same particular value.