What this means is for example, if you play Battlefield 4 on the Xbox 360, but later want to transfer all of your career and multiplayer rank over to the new Xbox One that you purchased, you will NOT be able to do so. You will be starting over again on the Xbox One version of Battlefield 4.
This isn’t really a change and is actually how it works with Battlefield 3 right now. So this does not come as a surprise actually. It would have been nice to have cross platform support, but it would have been difficult to implement that is for sure.