What is "Smart Standby"?
After some digging, I found a German site https://durcheinandr.de/technik/smart-standby-welch-eine-option 

To save you the hassle of translating, it states that enabling smart standby allows the device to automatically lock and turn the screen off when the phone is placed in your pocket, or on a flat surface.

I barely remember my German. God, that was hard.