You are standing next to a chimney, which used to be attached to one of the three crematoria of Mauthausen.
In the basement underneath you, there used to be the crematoria, a mortuary cooler, and rooms of execution.
If you want to look at these, you can gain access via the Museum after completing the IWalk. The permanent exhibition on the ground floor explores the four stages of the camp's evolution mentioned at the beginning. The Rooms of Remembrance can be found in the basement.