PvP Arenas are a battle mode where you use your dragons to fight other Dragon Masters for rewards. PvP Arenas are unlocked at level 11.
Arenas are element-specific: the element(s) indicated on each Arena mean that dragons with the same element will receive bonuses like health or damage boosts while battling.
Once you join the PvP Arenas, you will be automatically placed in the Beginner Arenas for you to work your way up.