Here is a very quick and simple guide to contour lines. It is just intended to get you started so please take the time to practise and learn how to recognise the shapes of hills from contours on the map. I think this is one of the best navigational skills to learn if you want to navigate in hills.
On an Ordnance Survey map at 1:25000 scale the contour lines are narrow brown lines (to be honest they look orange to me but I’m told they are brown). You can recognise some basic shapes of hills in the following way.
Hill
Average hill shape
Contour lines for a hill
What a hill actually looks like
Valley
Cross section through a valley
Contour lines for a valley (with a stream in the bottom)
What a valley looks like on a map
Saddle
A saddle – a dip in a hill
Contours for a saddle
What a saddle looks like on a map
Convex Slope
A convex slope – steeper at bottom
Convex contours – steeper at the bottom
What a convex slope looks like on a map
Concave slope
Concave slope – steeper at the top
Contours for a concave slope/hill
Concave slope on a map