It's a confirmation consisting of just one line.

I've never heard that phrase before! I'll give it my best guess, though. "One-liner" in this context probably means short, concise. Your response should be 'one line' of text. "Confirmation" means to verify, to give a response that you understood, or that you are able to do something, etc. So I guess "one-liner confirmation" means they want a short, 'yes' or, 'I understand' answer.

Thank you for your help! :)