For me it would be the following:
Your Hunter's Mark is now undispellable and increases the damage done to the marked target by 5% (or 10%). Additionally, your attacks against the marked target are no longer subject to line of sight restrictions.
This basically lets you shoot through any kind of obstacle the target is hiding behind, once it is marked. Ranged attacks already go through hills and so on, and in a world full of magic, even this would be possible. :-D