Conversation history is the earlier messages in a chat that an AI application can refer to while answering. It may include what you asked, what the AI replied, and information added by the app.
Think of it like keeping a few pages of a discussion on the table. If you ask “What about the second option?” the earlier messages can help the AI know which option you mean.
An app may not keep every message forever or send the whole chat each time. If a detail matters, repeat it clearly rather than assuming it will always remain in view.