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. With practice you can look at a map and envisonage the landscape as it should appear in real life. At the very least you should be able to tell if you have to walk up a mountain or if it is flat ground all the way. By applying a few simple principles you can learn a lot about the 'lay of the land' from contour lines. In maths we would call the steepness the gradient of the slope.