Drafts
Tap Save draft at any point and come back to the record later. Drafts sync like anything else, so a draft started on a phone can be finished on the web, or the other way round.
Resuming one
Open the form again and the app offers to resume the draft.

The Drafts tab lists every draft on the device, with the form it belongs to and when it was last edited.
Drafts do not last forever
A draft untouched for 30 days gets a notification saying it is going. If it is still untouched 14 days after that, it goes to the recycle bin — binned, not destroyed, so a manager can pull it back for another fourteen days after that.
Opening and editing a draft withdraws the warning and resets the clock.
A draft form is not a draft record
An unpublished form nobody can use yet and an unfinished record somebody means to return to both say “draft”. This page is about the second. The first is in Building forms.