The contour lines overlay is available to Footpath Elite users using the Footpath Topo map style. Free users can see basic contour lines, but the labels indicating the contour elevations are not displayed.
You can use this overlay to adjust the contour elevation labels for display in meters or feet.
Contour lines indicate the overall topography of the terrain. When the lines are spaced closer together, it indicates steeper terrain. In particularly rugged areas, you might be interested in the slope angle overlay.
You can find a detailed guide on how to read contour lines here.