Well, contour lines are used to signify a three-dimensional image of a flat surfaces. They are commonly used on topographic maps. Here contour lines connect continuous points of equal elevation. There is a connection between the change of height and the relative change of distance. If you move from one contour line to another then there is a change of height. The closer the lines are the 'faster' the height changes, and thus the 'steeper' the terrain. The further apart the contour lines are, the greater distance over which height changes, the slope is gentler.
