If you're privy to something, you've been let in on a secret or know about something that most people don't. Not many people are privy to what those top secret documents are all about, for example.
The adjective privy comes from the Latin privatus, meaning “private,” and describes someone who has knowledge of secret or confidential information. But watch out: If you use privy as a noun, it can mean “outhouse” or “toilet,” which actually might be a good place to hide your diary if you don't want anyone else to be privy to its contents.