I read Slaughterhouse Five some years ago but remember it well.
What Kurt Vonnegut Jnr has done here is take his central character: Billy Pilgrim, and given him an impossible viewpoint on life. Billy's consciousness (the now or present in which he lives) has come 'unstuck in time' so his life isn't lived like ours: from the past - through the present (the now) - to the future. His 'now' jumps about between different points in his life in a none linear fashion. This gives Billy insights into the human condition which aren't available to the rest of us who are 'stuck in the moment'.
You could describe this way of looking at things as being the literary equivalent of cubism wherein the artist imagines himself (herself) to be a box studying an artefact or model contained within the box. How would a box perceive an object contained within it?
The point Vonnegut is trying to make, I think, is that if you limit your viewpoint then your understanding of the world and everything in it will be - likwise - limited.