Arena is a vital part of gaining prestige in Bleach Online. You can gain Arena Ranks by challenging and fighting other players in Player VS Player combat (PvP for sort) in doing so you will rise in rank based on whether or not you defeated the player you challenged. Based on your rank is how much Prestige you will get Daily. More about Prestige and Ranks here.
While PvPing you will see several icons which are called buffs or debuffs. Buffs work in your favor giving you extra attack or speed. Debuffs will make you unable to attack, lower your defenses or poison you.
Anyone you defeat in the Arena can be taken as a slave. You can do so from your Home, but you can only take players as a slave if they are close to your main characters level.