in reply to frongt
Those are small plaques that are put in the pavement outside the houses of people that were deported by the Nazis. They usually carry the names of those people and the dates of their deportation. There is an ongoing effort to install those to give a name to the victims. The Nazis hate them.
in reply to Diplomjodler

They also serve to decentralize remembrance. A Holocaust memorial is good, yes, but it's in one place you're probably not going to go unless you want to. Stumbling stones can be randomly encountered while you go about your day and remind you that someone who had their life ruined once lived here.

That's precisely why right-wing politicians oppose them in favor of centralized memorials.