Well the trivial answer is that they are those faint brown lines all over your Ordnance Survey map.
Contour lines are a way of representing height and structure on a flat map.
Every point on that contour line is at the same height above sea level. You will find that not every contour line is marked with an actual value. Only a few contours are actually marked with a number. Contour lines are invaluble in certain situations. Walking, route planning, direction finding or position locating require a basic understanding of contour lines.