An energy-saving mode your Android device usually switches to when the display is off. Details can be found following the 'info' link below.
Android devices are powerful computers, and their most limiting part is the battery. In order to make the battery last as long as possible, Android is designed to save as much juice as possible: if there's nothing to do, don't do it!
A very special case here is the standby mode: with the display turned off, the system tries to minimize ressource usage as much as possible, switching off many components (such as e.g. WiFi and many network activities), setting CPU tacting to lower values, etc -- of course unless explicitly (by settings) or implicitly (by events) advised differently (you wouldn't love it if the music player suddenly stops just because the display turned off).
If you've got trouble with standby misbehaving, or your device never going to standby mode, this is the right tag for your question. Other tags might be related and maybe useful to have in addition (or instead of) the standby tag. A few examples would be:
- battery-life: If your device seems to "eat" too much battery in standby mode
- wifi: If you have issues with WiFi in standby (e.g. aborted connections, or WiFi not being turned off when it should be)
- music-player: If e.g. your music player has problems when the display is turned of (stuttering, or it even completely stops to play)
- connection-issues: General network problems in standby, e.g. connection loss to the mobile network
