Smart stay allows your Android device's screen to stay on for longer than your usual screen timeout if its front camera can see that you are still looking at the screen.
This feature is unique to certain Samsung Galaxy phones and tablets.
There are limited circumstances where this feature would be useful: Android already knows to keep the screen awake while you are watching a video. If you read a large amount of text, you will be regularly touching the screen or a button to scroll the text, which will also keep the screen active.
The feature may be useful in rare situations where you need to keep watching the screen, but it's not for a video or something that you are scrolling regularly.
This feature may work poorly in some situations. For example, if you are in a dark environment, or not holding the phone so that it is pointing at your face, it might not be able to see your face and may not work.
This setting may cause your device to consume more battery power due to its use of the camera.