The horse represents a spiritual or magical, mystical journey. The church represents the dreamer's "house", or unconscious. Going straight, veering neither left nor right, the horse is telling her not to become sidetracked, but that she has the "light" within herself, and only needs to "go straight" in order to turn it on. I think this means the light is not difficult to find within herself -- no special map or set of instructions is necessary. Right now, she has her unconscious "locked," but once she recognizes that the light is within herself, she will unlock the door and wonderful things can happen for her.
I hope this helps.