As illustrated, you have accessed the Task Switcher. Depending upon your iPad model, this screen is accessed in one of two ways.
1) For iPad models with a physical Home Button, a double-click of the Home Button will expose the Task Switcher.
2) For all models of iPad - both with or without a physical Home Button, swiping upwards from the bottom-edge of the screen to the centre - and pausing briefly before lifting your finger from the screen, will open the Task Switcher.
From here, you can quickly switch to any open App by tapping the associated thumbnail. Apps can be similarly force-closed by swiping the associated thumbnail off the top of the screen.
More information can be found here:
Switch between apps on iPad - Apple Support
The iPad User Guide is also a rich source of information about your iPad and iPadOS. 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