Contour lines are used in a map to portray differences in elevation.
When contour lines are closer together on a map, they indicate a steep slope. Think of contour lines as the distance between each incline. The closer together the inclines, the steeper the hill.
On the other hand, the farther apart lines tend to indicate a depression in the landscape.
Another way to tell elevation is the numbers on the map. Each number represents a different elevation, the higher the number, the higher the elevation (duh) and vice versa.