Smart Stay is a unique feature which is available in most of the Samsung Galaxy series. This feature is all about saving your battery life. When this feature is activated it uses the front camera to determine where your eyes are looking at. I.e. when you are looking at the phone it keeps the brightness as it as and when you look somewhere else it dims the brightness of the phone to give more life to the battery.
The phone screen stays when you are looking at the phone and when you are done it automatically dims the display and then the timeout locks the screen.
To activate this feature you could either go to Settings or the Notification panel. From the Notification panel tap on the Smart stay option as shown in the below screenshot. Once the Smart Stay feature is enabled you could see the eye icon on top of screen before the sim1 slot.