This is known as a Sarajevo Rose, a concrete scar caused by an artillery explosion that was later filled with red resin. They mark the areas where people were killed.
Sarajevo was a central zone of conflict during the Bosnian War. The Bosnian Serb Army deployed troops and artillery in the surrounding hills, and on May 2, 1992 began imposing a blockade on all traffic in and out the city, starting what was to be known as the Siege of Sarajevo. They constantly bombarded the civilian population for nearly four years, launching an estimated 300 rounds of artillery every day.
I chose this particular one because of the context: young people casually walking by a place where some years ago, a human life was lost.