From you: your name, your restaurant's name, a phone number and an email, so somebody can ring you back.
From your restaurant: your menu, your tables, your bills and your takings. That is your business record and it is held so you can read it back.
From your guests: nothing, unless you type it. There is a name and phone field on a bill for a delivery order and it is optional.
An enquiry sent from this website reaches a named person here with a date to reply by. It is not passed to anybody else.