as you fight for your faction in Alterac Valley, you increase your reputation with the Horde or Alliance NPCs in the Battleground. As you do so, you will unlock special rewards available only to the most reputable of players. There are two special vendor NPCs, one for Horde and one for Alliance, standing outside of their respective entrance to Alterac Valley. These two NPCs sell special rewards. When you raise your reputation level with them to a new category, you unlock a new set of rewards. For example, once you reach friendly reputation, a page of secret items is revealed to you. When you reach the next category of reputation, another page opens for you. You can continue to improve your reputation with your faction up to exalted status, and continue to unlock more pages of rewards. You can see the special Alterac Valley rewards below.
Remember that these rewards are in addition to the general PVP Honor Rewards you earn by improving your rank as you participate in Battlegrounds.