Co znaczy Sunken Kerbstone,
I saw it while practicing for driving lisence in Germany! ?

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I do suppose in German, it means "abgesenkter Bordstein".
Sidewalks are usually a few centimeters above street level, and on their street-side end, they have a kerbstone or "Bordstein" - here's an image:
https://vorort.news/Uploads/2fbab1d1a52fb6e4d04...

In places where cars will have to drive across a sidewalk (like, at a driveway leading up to a garage) or where people in wheelchairs may have to cross from one to the other side of the street, that kerbstone is often lowered. That way, it becomes easier for cars/wheelchair-bound people to get over the kerbstone. Here's an image:
https://blog.lapid.de/hubfs/Marketing/Blog%20Me...

They also signal who has priority in driving into a street. If your car has to cross an "abgesenkter Bordstein", you have to give other cars priority before you can do so.