It’s a little button that pops up in the bottom left corner when you’ve rotated your phone but have auto rotate off on newish versions of Android. If you push it, it rotated to landscape or portrait depending on what you’re already in. It’s animated and shows an icon of a phone rotating, which can be quite distracting at first or on certain flavours/brand implementations of Android. It animated about 3 times in stock Android before sticking around for a few extra secs then vanishing until you rotate back to your chosen orientation and back