Offline or Stealth Mode is a way to hide your online status in the League of Legends client. We use it at your request and it's free. Thanks to hiding, the boosters do not have access to the friends list, and your friends cannot see that we are using the account (we are offline for them, so we can easily boost when you are at school or work). It also means that you can play other games on your account, or use your phone at the same time to chat with your friends. Offline mode is simply adding a LoL chat server to your firewall.