Expertise increases players' effectiveness in PvP combat by enhancing their damage, damage reduction and healing against other players.
Players do not have a base stat for Expertise. To gain Expertise, a player must equip PVP armor that has an Expertise bonus. However, players are not able to purchase said PVP armor from PVP vendors until they reach the higher Valor ranks. Those that haven't achieved the high level ranks can use Expertise stims to temporarily increase their Expertise stat by a small amount. These stims are available from PVP vendors.