A repositioning cruise is one where the ship goes east to west or west to east and stays at that port. For instance; It is spring time and a ship wants to sail in Alaska. So they post a special going from say Ft. Lauderdale to Vancover, that means you can sail from Florida, thru the panama canal, up the west coast to canada. The only problem will be to fly back home, which can be expensive since it it one way. Airlines like to give you better rates for round trip.
Hope that helps. We are looking for this kind of trip too. Let me know what you find out, maybe we can be on the same one??