You have encountered the iPadOS “Shelf” - this being a feature of the revised iPadOS multitasking environment.
iPadOS permits you to have multiple “instances” (i.e., App windows) of the same App - each instance being self contained. The shelf, when triggered, shows a thumbnail of each running instance of an App.
For example, if you have three running instances of Safari, each with its own set of open tabs, you will see three thumbnails corresponding to each running instance of Safari - plus a button to open a new instance of Safari. Simply tapping a thumbnail will switch directly to that running instance of Safari. Each App - such as Mail, Safari, Notes, Calendar, etc. - has its own corresponding shelf.
Individual App windows can be closed directly from the Shelf; just swipe the unwanted thumbnail off the top of the screen.
If you are running iPadOS 17, it is possible to completely disable iPadOS multitasking - and in so doing completely inhibit the appearance of the shelf:
Settings > Multitasking & Gestures > [Multitasking] select OFF

You should note that the Shelf cannot be disabled in isolation.