This is the 5964th most frequent Russian word.
секретных
"Secret" or "classified."
The word 'секретных' is used as an adjective describing the research ('исследований') as being secret.
Она нашла документы с секретных исследований.
She found documents from secret research.
Here, 'секретных' is an adjective modifying the noun 'данных' to indicate that the data is of a classified or confidential nature.
Секретных данных не должно быть доступно всем.
Secret data should not be accessible to everyone.
'Секретных' is used to describe the materials ('материалах') as classified or confidential.