This is in the front entrance of a very large psychiatric institute near where I live, and it is one of the Icons of the National Socialist era in this region.
The "grey buses" were used to take people from institutions for mentally and psychologically disabled people in this region as part of the "Euthanasia programme" in 1940-41.
Where the buses took people away they did so with the assistance of the institutions management. We know this because some refused to cooperate, and when they did the officials backed down and left.
The alternative title of the memorial is the question many patients asked when they were herded onto the buses: "Where are you taking us?"
The memorial is relatively new, and as far as I two buses have been made: this bus "tours" various places where people were taken away. The other has been placed to permanently block one of the entrances to a building where the deaths were planned. Germany is determined that his time won't be forgotten or repeated again.