Peek (aka: Aero Peek) is the feature responsible for Peek at desktop and taskbar thumbnail live previews.
Peek at desktop has been removed from the Show Desktop button on the taskbar in Windows 11, but you can still press the Win+, (comma) keys to temporarily peek at the desktop.
When you hover the pointer over an icon on the taskbar with an open window, you will see a thumbnail preview of the open window. When you hover the pointer on the taskbar thumbnail preview, you will see a live preview (peek) of the open window on your desktop. This is a taskbar thumbnail live preview.
This tutorial will show you how to enable or disable Peek for your account in Windows 11.