What does a gentle slope on a topographic map represent?

The US Geological Survey's topographic maps have "contour lines" showing elevation. The farther apart the contour lines are, the more gentle the terrain is; the closer together they are, the steeper the terrain is. The darker contour lines typically denote 100-foot elevation changes and are marked as such, while the lighter lines typically indicate 10 or 20-foot changes in between. The interval can easily be determined by counting the number of lighter contour lines between the darker lines. The steepness of the slope can be determined by using the map's scale (which is indicated at the bottom of the map) to measure the distance between two points and using the contour lines to see the change in elevation.