This is the 5963rd most frequent Russian word.
секретной
Secret.
The word 'секретной' is used as an adjective describing the folder, indicating its confidential nature.
Эта информация содержится в секретной папке.
This information is contained in a classified folder.
Here, 'секретной' modifies the noun 'миссия' to denote that the mission is secretive and not publicly disclosed.
Он работает над секретной миссией.
He is working on a secret mission.
The term 'секретной' describes the laboratory, highlighting that it is private or concealed from general knowledge.