To save a form for later, tap Back in the top-left corner. Your entries are kept and the form appears under Forms In Progress on the Forms screen, with an orange bar.
To resume, go to Forms in the ☰ menu and tap the form under Forms In Progress. It opens where you left off.
To delete a form in progress, swipe left on it in the Forms In Progress list and confirm when prompted. This removes the form and everything you've entered — it can't be recovered.
Forms in progress are stored only on your iPad. They aren't visible to your organisation until submitted, and they will be lost if you log out. See Logging out.