This is the 4752nd most frequent Russian word.
закрытом
"Закрытом" means "closed" (in the prepositional case).
The word 'закрытом' here describes the attribute of the 'помещении' (premises) as being closed or enclosed.
Я был на мероприятии в закрытом помещении.
I was at an event in a closed indoor space.
'закрытом' describes the state of the 'ящике' (box) as being closed.
В закрытом ящике лежали важные документы.
Important documents were lying in the closed box.
In this sentence, 'закрытом' metaphorically describes the nature of the 'обществе' (society) as being exclusive or restricted.