They are called Stolpersteine (stone on the ground that can cause one to trip) and they have a very important meaning. They are cobblestones that form part of a project initiated by Gunter Demnig to commemorate all those who were deported and killed by the National Socialist Party (Nazis). What we see in the photo is a cement cube 10 x 10 x 10 centimeters which at the top has a brass plate in which the essential details of the person commemorated are engraved. The idea is that when you are walking if you trip over a cobblestone you will lean forward to see what you tripped on and maybe read the plate. This inclination could count as a gesture of respect for the person who is remembered. But the basic idea is to not let what happened be forgotten. The Stolpersteins can be found all around Germany. The goal is to install as many as possible, there are several organizations that investigate who were the victims and they work on starting the paperwork for the Stolpersteins creation and installation.