The term "parallel to horizon" means that something is running in the same direction as the horizon. The horizon is the line that separates the earth from the sky. It is parallel to the ground and appears as a straight line when viewed from a distance. Objects or lines that are parallel to the horizon will also appear as straight lines, and they will run in the same direction as the horizon. For example, the surface of a calm lake may appear parallel to the horizon, as would a long, straight road stretching out into the distance.