This part was sung in the "Frozen" movie. There was a pause between "is-" and "-olation," which made it at first sound like "A kingdom of ice." Really cool double entendre.
This part of the song means that her kingdom is separate from everything, everyone so, she made it that no one can find her.
Kingdom of isolation. Place of her own zone where she kept all her fear and everything about her, side that no one has ever known. Place that separated her with her beloved sister so she could keep her safe.