This is the 798th most frequent Serbian word.
отвореном
"Отвореном" means "open" in the instrumental case, often translating to "in the open" or "outdoors."
The word 'отвореном' here is in the instrumental case, agreeing with 'простору' to describe the type of space.
Провели смо цео дан на отвореном простору.
We spent the whole day in the open space.
The word 'отвореном' is used adverbially to indicate that the conversation took place outdoors.
Разговарали смо на отвореном о нашим плановима.
We discussed our plans out in the open.
Here, 'отвореном' functions as a locative case noun descriptor for context of being outside.