Surface normal
An arrow pointing straight out of a surface, used to work out how light falls on it.
See it in Depth terrain. Try a sample map, then set Render mode to Normals.
In plain words
Every spot on a surface faces some direction. The normal is an arrow pointing straight out from that spot. If the normal points toward the light, the spot is bright. If it points away, the spot is dark. Normal views paint these arrows as colours, so slopes facing one way share one colour.