noun which may take the genitive case particle 'no', noun
reserved; booked; already taken
Indicates that a seat, room, ticket, or other resource has already been reserved and is not available. Often used in signs, online forms, or when checking availability.
この席は予約済みです。
This seat is reserved.
ホテルの部屋はすべて予約済みだった。
All the hotel rooms were already booked.
満席 means 'full house' or 'all seats taken', while 予約済み specifically means 'reserved' (possibly not yet occupied).
Compound of 予約 (reservation) + 済み (finished, already done). The suffix 済み attaches to nouns to indicate completion of the action.