How does a topographic map show terrain features?
Contour lines show the rise and fall of elevation.
Among many other things, a primary feature of topographical maps are contour lines. Here's a quick and dirty illustration:
So, imagine this as a hill that, from the bottom left of the diagram proceeding up, gains elevation. You can think of the distance between each line as 20 meters up or down, depending on the direction you're going. So the first line after 100m is 120 meters above sea-level, then 140, then 160, 180, and finally 200m.
Another way to think of it would be if you were to walk along the contour lines perfectly, you would never gain or lose elevation!
Also, each topo map has a different measure between contour lines (20m is actually pretty big compared to most maps I've seen, though I live in a pretty flat place!) that's usually printed somewhere on the map.