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.
When is this useful?
In typical usage, this feature is probably rarely needed. Android already knows, for example, if a video is playing and won't turn off the screen automatically if a video is still playing.
If you are reading a large amount of text, in most cases you'll be regularly touching the screen in order to scroll through the text or move to a new page, 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. Or, perhaps you have an app that plays video or displays some live information, and for some reason it has not told Android to keep the screen awake while it does so.
How effective is it?
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 while the screen is on.
What are the privacy implications?
This feature does not require images taken from the camera to be sent to the internet or shared with any company, including Samsung or Google. The processing of imagery from the camera is done on the phone itself, and there is no reason for the images to be stored on the phone. As long as you trust that Samsung does not alter this behaviour, there is no reason to assume imagery taken by the camera in this way is likely to make it to anyone else.