That is the before/after button. Press your finger on it and hold to see the "before" image without adjustments, release your finger to see the "after" state with adjustments.
I think it's pretty clever that they implemented it that way, momentary rather than as a toggle - so we don't get people toggling the "before" state and wondering why they can't see any adjustments
Thanks, I knew it was something basic like that. Handy to have...