You need to explore multitasking - as you have a slide-out view of an App (in this case Safari) enabled. Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.
This button is a core element of the iPadOS15 User Interface - intended to simplify use of full/split/slide-over multitasking. This control cannot be hidden or suppressed - and neither can multi-tasking be disabled in iPadOS15.
More information about using multi-tasking can be found in this support page:
Use multitasking on your iPad - Apple Support
The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892