Assuming you are referring to the Overview button in Android 9.0 and up:
The Overview button, also known as the Recent Apps button, is a key feature in Android that allows users to quickly access recently used apps. When pressed, the Overview button displays a list of thumbnails for all open apps and windows. This can be helpful for quickly switching between apps or accessing recently used files.
There are two ways to access the Overview button: by swiping up from the home screen or by tapping the icon in the navigation bar. The behavior of the Overview button can be customized in Settings -> System -> Gestures -> Swipe up on home screen. By default, this gesture will display the app drawer when no other windows are open. However, it can be changed to always show the overview screen regardless of whether any other windows are open.
Android 9.0 also introduced a new way to access recent apps: by holding down the overview button. This will display a list of your most recently used apps, allowing you to quickly switch between them. You can also swipe left or right on this list to see even more recent apps.