In this sentence, the dream is described as a goal or what u want. otherwise known as not the dreams u have in ur sleep. this means that nightmare isn't the bad dream u have in ur sleep. the nightmare in ur sleep is just something bad happening that wont be true ( usually ). the nightmare this question asks is the opposite of a goal.
nightmare is something you dont want to happen. something that stresses u out or makes u scared and uncomfortable. something u fear will happen or something that happens right now that u feel makes u uncomfortable.
hope this helps a bit! :)