Runoff is a compound word. Let us break it apart.
Run - To move,
Off - A direction. Away from, not on.
So runoff means - to move away from, or to move off in a direction.
So surface runoff means something that (moves away from the surface) or in this case, runoff. When combined with surface, it is usually referring to water. Rain is water. Some of it goes into the ground, the rest does not, and creates runoff :)