I'm not sure, but; I think that was actually an "Incinerator."
When I was a kid back in the 1960's here in Denver, Colorado;
every house had one of those in the back yard.
It was for burning paper, and other trash.
Then one day a Law was passed (I think) making them illegal to
use anymore; all trash had to go to the Dump.
Or; it really could be a real cooking pit someone made there
many years ago. Either way it looks like it works great for
doing some wicked Q.