The horizon was once considered to be the edge of the world and in dreams can still have that connotation, though now it is the boundary of the visible physical world. Beyond that is the Abyss or Void. When a new idea strikes and we become conscious of the narrowness of our viewpoint, we may literally dream of a widening of our horizons. Our perspective changes. In everyday life when change is the order of the day we will become conscious of changes on the horizon in our dream - perhaps a new building or other object - which may need interpreting, to give us clarity in our prospective actions. You might also like to consult the entries for Abyss, Globe and World.