Mysterious floating button on iPhone after iOS 26 update

Upon installing, from first unlocking of iPhone, this appeared and remains

So what is this button for. It moves when dragged, can change screens like apps are dragged, sits over and blocks desktop; press and it changes to semi-transparent grey but doesn't appear to actually do anything. 

Thanks d

[Re-Titled by Moderator]

Original Title: Spare Button or? IOS26