A form is the question set a client fills in when they book. Every account starts with one undeletable Default form, four questions always on (first name, last name, phone, email) plus three you can turn on or off: which licence they hold today (G1/G2/Full G), when they got their G1, when they got their G2.
You can add your own questions too, in plain text, yes/no, or multiple choice, and reorder or turn any of them off. Deleting a custom question you’ve already collected answers for doesn’t erase those answers; it just stops asking new clients, and the old answers stay on each client’s record.
A form only applies once it’s attached to an appointment type, which happens on the Appointments screen, not here. You can build a form and leave it unattached; nothing breaks, it just isn’t shown to anyone yet.
Turning off the G1/G2 date questions doesn’t turn off your reminders. They still need those dates to fire, from whichever source has them (this form, a CSV import, or a manual add). Switching the questions off just means you’ll be filling those dates in yourself instead of a client supplying them at booking.