https://www.cnbc.com/2018/07/30/how-to-turn-off-zoom-on-iphone-weird-rectangle.html

There's an Apple iPhone feature that you might turn on by accident from time to time. As you can see in the picture above, it just looks like a rectangle that sits on your display and gets in the way of everything.

This rectangle actually has a purpose. It's called "Zoom" and is used to magnify certain parts of the screen. If it's active, the rectangle will appear if you double tap the screen three fingers.