Sealed bottle is a quest item in The Witcher 3: Wild Hunt that contains Johnny's "voice" so that he can't talk. He apparently lost it because he spoke badly of the Crones who then decided to punish him by taking his voice and bottling it up.
Geralt must retrieve this to get Johnny to talk again so he can tell Geralt what he knows about Ciri. It can be obtained prior to actually finding Johnny, whereupon it'll sit in the quest items inventory until needed.