This is the rail entrance to Birkenau, the largest killing center in the entire Nazi system. Beyond the gate is what is generally known as “the ramp”.
If you continue to look through this site, you will see other pictures of it from inside.
It is where Jewish people mainly, men women and children, were unloaded and sent either directly to the four gas chambers here, or to slave labor.
This particular ramp was used mostly to dispose of the Hungarian Jews in the spring and summer of 1944.