This is the 3293rd most frequent Russian word.
открытых
"Open" or "of open" (genitive/plural form).
In this sentence, 'открытых' (open) is used as an adjective describing 'площадках' (spaces or venues), indicating that these venues are outdoors or not covered.
Мы провели встречу на открытых площадках.
We held the meeting in open spaces.
Here, 'открытых' (open) modifies 'вопросах' (questions), meaning unresolved or ongoing topics in philosophy.
На столе лежала книга об открытых вопросах философии.
On the table lay a book about open philosophical questions.
'Открытых' (open) describes 'дверей' (doors), specifying that the doors were not open.