Have you ever used a "geodetic map"? Those are maps that show, in addition to highways and towns, hills, trails, etc. so are used by hiker, hunters, and the like. They are called geodetic maps because they also show "geodetic levels", lines of constant altitude. That is precisely what contour maps are: the lines show constant height of the three dimensional figure at two dimensional (x,y) points. Since the contour lines, like the geodetic lines, are shown at a constant interval of height, if two lines are close together, that means you have to go up that distance in a short horizontal distance. If two lines are farther apart you have to go up that distance in a longer horizontal distance. In other words, the closer contour lines are together the steeper the slope is there. On a geodesic map, lines that are far apart means a nice gentle slope, line that are very very close together means a cliff!
