The lines on these maps are called contour lines.  They connect points of equal elevation.  If you were to travel along one of these lines, you would stay at the same height above sea level. 

Every fifth line on a topographic map is an index contour and has its elevation labeled.  On this map, the 2500 contour line is darker because it is an index contour.  The 2400 and 2600 contours are also darker.

Contours