Ordnance Survey maps use contour lines to join points of equal height together. Understanding contours is a very useful navigation skill because you can identify the lay of the land and landscape features as they appear on the ground. They tell you whether the ground is flat, hilly, undulating, or steep, and whether a route will be a gentle easy walk or a hard uphill slog, so you can plan your route more easily.
Contours are shown on Ordnance Survey maps as thin orange or brown lines with numbers on them that show you the height above sea level of any point on the line. The closer the contour lines are together, the steeper the slope. Contour lines very close together indicate a steep slope and contours further apart show a gentle slope.