。
Your reservation has been confirmed.
Literal
Reservation [topic-は] confirm-[passive-されている].
~されている stacks the passive (confirm-[passive]) onto the resultant-state ~ている — 'is [currently] in the state of having been confirmed.' This is the standard way to report an already-completed passive action in Japanese. 確認する ('to confirm') is a suru-verb taking 確認 as its noun stem.