When someone taps your WhatsApp button, Roazr can show a small popup that asks for their name (and their WhatsApp number, if you have turned that on). Form Mode is the setting that decides when that popup shows up.
There are three options: Auto, Form always and Skip always. Auto is the default, and it is the right choice for almost every seller. This page explains what each one does, and what you give up when you turn the popup off completely.