Screen saver / Daydream
A new feature introduced in relatively recent versions of Android, this feature shows a decorative screensaver on the screen when it is asleep but the phone is plugged into mains power.
To conserve battery, this feature is never activated when the device is not plugged in to mains power.
This feature is intended for people who "dock" their device in a stand that provides power.
More settings can be accessed from the overflow menu (three dots).
Show the screen saver immediately. This can be used for testing your screen saver.
When to start screen saver
This option exists on some devices and allows you to specify whether you want to display the screen saver any time your device is connected to a charger, or only when docked (or vice versa).
It is not possible to configure the screen saver to come on when it is not docked or charging, except for the above Start now option which can be used for testing purposes. Displaying images on the screen uses a relatively large amount of power, and would drain the battery quickly if allowed when not connected to external power.