* What they are: Contour lines are lines that connect points of equal elevation on a map.

* How they work: Imagine slicing through a landscape with a series of horizontal planes. Each slice would intersect the terrain at a specific elevation. The contour lines show the outlines of those slices on the map.

* Interpreting them:

* Closer lines = steeper slope: The closer the contour lines are together, the steeper the slope of the terrain.

* Farther lines = gentler slope: Wider spaces between contour lines indicate a more gradual incline.

* Closed circles = depressions or peaks: A closed circle of contour lines indicates a hilltop or a depression (like a crater or a valley).

* Numbers on lines: The numbers on contour lines indicate the elevation in feet or meters.

In essence, contour lines are like a "level" or "elevation slice" representation of the terrain. They provide a visual way to understand the shape and elevation changes of the land.